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Slack is a cloud-based proprietary instant messaging platform developed by Slack Technologies.

Slack Technologies Logo.svg
Original author(s)Stewart Butterfield, Eric Costello, Cal Henderson, and Serguei Mourachov[1]
Developer(s)Slack Technologies
Initial releaseAugust 2013; 6 years ago (2013-08)[2]
Stable release(s) [±]
iOS19.11.20 / November 18, 2019; 21 days ago (2019-11-18)[3]
Android19.11.20 / November 18, 2019; 21 days ago (2019-11-18)[4]
Windows4.2.0 / November 26, 2019; 13 days ago (2019-11-26)[5]
macOS4.2.0 / December 4, 2019; 5 days ago (2019-12-04)[6]
Linux (Beta)4.2.0 / December 5, 2019; 4 days ago (2019-12-05)[7]
Written inElectron (C++, JavaScript, etc.)[8]
Operating systemMicrosoft Windows, macOS, Linux, iOS, Android, Windows Phone[9]
TypeCollaborative software[10]


Slack began as an internal tool for Stewart Butterfield's company Tiny Speck during the development of Glitch, an online game.[11][12] Slack launched in August 2013.[13]

In March 2015, Slack announced it had been hacked over four days in February 2015, and that some user data were compromised. The data included email addresses, usernames, hashed passwords, and in some cases, phone numbers and Skype IDs users had associated with their accounts. Slack added two-factor authentication to their service in response to the attacks.[14]

Slack was previously compatible with non-proprietary Internet Relay Chat (IRC) and XMPP messaging protocols, but the company closed the corresponding gateways in May 2018.[15]

"Slack" is a backronym for "Searchable Log of All Conversation and Knowledge."[16][17]


Slack offers many IRC-like features, including persistent chat rooms (channels) organized by topic, private groups, and direct messaging.[12] Content, including files, conversations, and people, is all searchable within Slack. Users can add emoji buttons to their messages, on which other users can then click to express their reactions to messages.[18]

Slack's free plan allows only the 10,000 most recent messages to be viewed and searched.[19]


Slack teams allow communities, groups, or teams to join a "workspace" via a specific URL or invitation sent by a team admin or owner. Although Slack was developed for organizational communication, it has been adopted as a community platform, replacing message boards or social media groups on Facebook or LinkedIn.[20]


Public channels allow team members to communicate without the use of email or group SMS (texting). Public channels are open to everyone in the workspace.

Private channels allow for private conversation between smaller sub-groups. These private channels can be used to organize large teams.

Direct messages allow users to send private messages to specific users rather than a group of people.[21] Direct messages can include up to nine people. Once started, a direct message group can be converted into a private channel.


Slack integrates with many third-party services and also supports community-built integrations. Major integrations include services such as Google Drive, Trello, Dropbox, Box, Heroku, IBM Bluemix, Crashlytics, GitHub, Runscope, Zendesk,[22][23] and Zapier.[24] In December 2015, Slack launched their software application ("app") directory, consisting of over 150 integrations that users can install.[25]

In March 2018, Slack announced a partnership with financial and human capital management firm Workday. This integration allows Workday customers to access Workday features directly from the Slack interface.[26][27]


Slack provides an application programming interface (API) for users to create applications and automate processes, such as sending automatic notifications based on human input,[28] sending alerts on specified conditions, and automatically creating internal support tickets.[29] Slack's API has been noted for its compatibility with many types of applications, frameworks, and services.[28]


Slack provides mobile apps for iOS and Android in addition to their Web browser client and desktop clients for macOS, Windows, and Linux (beta). Slack is also available for the Apple Watch, allowing users to send direct messages, see mentions, and make simple replies.[30] It was featured on the home screen of the Apple Watch in a 2015 promotional video.[31] Slack has been made to run on a Super Nintendo Entertainment System via Satellaview.[32][33]

Business modelEdit

Slack is a freemium product, whose main paid features are the ability to search more than 10,000 archived messages and add unlimited apps and integrations. They claim support for an unlimited number of users. When freeCodeCamp attempted to switch its community of over 8,000 users to Slack in 2015, however, they experienced many technical issues and were advised by Slack support to limit their channels to "no more than 1,000 users (ideally more like 500)".[34][35] That specific limit no longer applied by January 2017.[36]

On July 26, 2018, Atlassian announced the shutdown of its competing HipChat and Stride, effective February 11, 2019, and the sale of their intellectual property to Slack, with Slack to assume the user bases of the services. The companies also announced a commitment to work on integration of Slack with Atlassian services.[37][38]


8,000 customers signed up for the service within 24 hours of its launch in August 2013.[13][39] In February 2015, the company wrote that around 10,000 new daily active users were signing up each week, and had more than 135,000 paying customers spread across 60,000 teams.[40][41] By April 2015, those numbers had grown to 200,000 paid subscribers and a total of 750,000 daily active users.[42] Late in 2015, Slack passed more than a million daily active users.[43][44] As of May 2018, Slack had over 8 million daily users, 3 million of whom had paid accounts.[45] At the time of its S-1 filing for IPO, dated April 26, 2019, Slack reported more than 10 million daily active users from more than 600,000 organizations, located in more than 150 countries.[46]

The Financial Times wrote in March 2015 that Slack was the first business technology to have crossed from business into personal use since Microsoft Office and the BlackBerry.[47] In 2017 a writer at New York magazine described it as "a compulsion, a distraction[, a] burden ... another utility we both rely on and resent".[48]

Slack was recognized as the best startup of the year 2017 at the 10th Crunchies Awards, organized by TechCrunch.[49]

The digital rights group Electronic Frontier Foundation (EFF) has cautioned that "Slack stores and is able to read all of your communications, as well as identifying information for everyone in your workspace".[50] While commending the company for "follow[ing] several best practices in standing up for users" concerning government data requests, such as requiring a warrant for content stored on its server, and awarding it four out of five stars in its 2017 "Who has your back" report,[51] the EFF also criticized Slack for "a broad set of exceptions" to its promise to notify users of such requests, and for other privacy shortcomings.[50]


Slack has been criticized by users[52] for storing user data exclusively on cloud servers under Slack control.[53][54]

This is found to be a particular issue for users with large teams, who experienced issues with connectivity within the app, access to archived messages, and the number of users for a given "workspace".[55] Slack has additionally been criticized for a recent change to their privacy policy, allowing access to all public and private channels by workspace administrators, without the need of consent from any parties using the app.[56] According to the policy, Slack users would not be notified when their information is being accessed.[57] Other notable issues include being criticized as addictive,[58] an inhibitor to productivity,[59] and showing personal information such as email to other users by default.[60]


Open sourceEdit

There are a number of free and open-source software alternatives or protocols to Slack that can be self-hosted, such as:


Other alternatives having a similar business model to Slack are Flock, Microsoft Teams,[61] Hangout Chats,[62] Symphony Communication, Trillian, Workplace by Facebook, Discord, Glip, Cisco Webex Teams, and Yahoo Together (now defunct[63]).

See alsoEdit


  1. ^ Kumparak, Greg (February 5, 2015). "Slack's Co-Founders Take Home The Crunchie For Founder Of The Year". Retrieved October 13, 2018.
  2. ^ Zax, David. "Flickr Cofounders Launch Slack, An Email Killer". Retrieved October 13, 2018.
  3. ^ "Slack - Team Communication". App Store. Apple Inc. Retrieved December 6, 2019.
  4. ^ "Slack for Android - Release Notes". Slack. Retrieved December 6, 2019.
  5. ^ "Slack for Windows - Release Notes". Slack. Retrieved December 6, 2019.
  6. ^ "Slack". Mac App Store. Apple Inc. Retrieved December 6, 2019.
  7. ^ "Slack for Linux (Beta) - Release Notes". Retrieved December 6, 2019.
  8. ^ "Desktop Application Engineer". Archived from the original on September 13, 2018. Retrieved October 13, 2018.
  9. ^ Slack. "Slack apps for computers, phones & tablets". Retrieved October 13, 2018.
  10. ^ "Crunchbase - Slack Technologies". Crunchbase. Retrieved October 13, 2018.
  11. ^ Tam, Donna. "Flickr founder plans to kill company e-mails with Slack". Retrieved October 13, 2018.
  12. ^ a b Thomas, Owen (August 14, 2013). "Die, Email, Die! A Flickr Cofounder Aims To Cut Us All Some Slack". Retrieved October 13, 2018.
  13. ^ a b Koetsier, John (August 15, 2013). "Flickr founder Stewart Butterfield's new Slack signed up 8,000 companies in 24 hours". VentureBeat. Retrieved October 13, 2018.
  14. ^ Greenberg, Andy (March 27, 2015). "Slack Says It Was Hacked, Enables Two-Factor Authentication". Condé Nast. Retrieved October 13, 2018.
  15. ^ Sharwood, Simon (March 9, 2018). "Slack cuts ties to IRC and XMPP, cos they don't speak Emoji". Retrieved October 13, 2018.
  16. ^ Kim, Eugene (September 27, 2016). "Slack, the red hot $3.8 billion startup, has a hidden meaning behind its name". UK Business Insider. Retrieved October 13, 2018.
  17. ^ "Never! You are correct, it is a backronym". Twitter. June 15, 2017. Retrieved September 27, 2019.
  18. ^ Crook, Jordan (July 9, 2015). "Slack Adds Emoji Reactions". Retrieved October 13, 2018.
  19. ^ "Message and storage limits on the Free plan". Slack Help Center. Retrieved October 13, 2018.
  20. ^ Diederichs, Matt (July 8, 2015). "Why Slack is Exploding (as a Community-Building Platform)". Medium. Retrieved June 18, 2019.
  21. ^ Slack. "Features - Slack". Retrieved October 13, 2018.
  22. ^ Gannes, Liz (August 14, 2013). "Flickr Co-Founder Stewart Butterfield Turns to Workplace Communication Tools With Slack". Retrieved October 13, 2018.
  23. ^ Augustine, Ann (May 19, 2018). "A Review of the Slack Communication Service". Retrieved October 13, 2018.
  24. ^ "Zapier | The easiest way to automate your work". Retrieved October 13, 2018.
  25. ^ Newton, Casey (December 15, 2015). "Slack launches an app store and an $80 million fund to invest in new integrations". Retrieved October 13, 2018.
  26. ^ Coop, Alex (March 26, 2018). "Workday and Slack announce partnership; Google, Microsoft and Facebook on the horizon". IT Business. Retrieved October 13, 2018.
  27. ^ News Desk, The HRT (May 27, 2018). "Industry Leaders Slack & Workday are Now Partners". Retrieved October 13, 2018.
  28. ^ a b Hotson, Dennis. "Building an Intelligent Bot Using the Slack API". Nordic APIs. Retrieved December 28, 2018.
  29. ^ "Slack API". Slack. Slack. Retrieved December 28, 2018.
  30. ^ Novet, Jordan (April 20, 2015). "Slack brings its app to the Apple Watch (video)". VentureBeat. Retrieved October 13, 2018.
  31. ^ Bort, Julie (March 9, 2015). "5 Apple Watch apps that will help you do your job". Business Insider. Retrieved October 13, 2018.
  32. ^ Birnbaum, Ian (October 22, 2018). "You Can Now Receive Slack Messages Inside a Super Nintendo Game". Vice Media.
  33. ^ Fan, Bertrand (October 18, 2018). "Slack on a SNES".
  34. ^ "So Yeah We Tried Slack... and We Deeply Regretted It". June 21, 2015. Retrieved October 13, 2018.
  35. ^ Kim, Eugene (June 22, 2015). "Startup founder claims $2.8 billion startup Slack is misleading people about its free 'unlimited' plan". Retrieved October 13, 2018.
  36. ^ Chen, Dave (January 20, 2017). "Despite its marketing, Slack's free tier limits your total number of users". Retrieved October 13, 2018.
  37. ^ Etienne, Stefan (July 26, 2018). "Slack buys HipChat with plans to shut it down and migrate users to its chat service". The Verge. Retrieved October 13, 2018.
  38. ^ Kumparak, Greg (July 26, 2018). "Atlassian's HipChat and Stride to be discontinued, with Slack buying up the IP". TechCrunch. Retrieved October 13, 2018.
  39. ^ Fingas, Jon (August 14, 2013). "Flickr creator takes sign-ups for Slack, an office collaboration tool with universal search". Retrieved October 13, 2018.
  40. ^ Griffith, Erin (February 12, 2015). "Slack growth skyrockets: 10,000 new active users each week". Retrieved October 13, 2018.
  41. ^ Kim, Eugene (February 12, 2015). "Billion-dollar startup Slack says it's adding $1 million in new contracts every 11 days". Retrieved October 13, 2018.
  42. ^ Welch, Chris (April 16, 2015). "Slack continues huge growth, is now valued at $2.8 billion". Retrieved October 13, 2018.
  43. ^ Metz, Rachel (June 25, 2015). "Slack Keeps On Growing". MIT Technology Review. Retrieved October 13, 2018.
  44. ^ Novet, Jordan (October 29, 2015). "Slack launches user groups, hits 1.7M daily active users and 470K paid seats". Retrieved October 13, 2018.
  45. ^ Newton, Casey (May 22, 2018). "Slack adds action buttons to become a true workplace communication hub". The Verge. Retrieved October 13, 2018.
  46. ^ "Form S-1 Registration Statement". Retrieved May 7, 2019.
  47. ^ "Slack: workplace message app so cute you want to use it at home". Financial Times. March 25, 2015. Retrieved October 13, 2018.(subscription required)
  48. ^ Fischer, Molly (May 17, 2017). "What Happens When Work Becomes a Nonstop Chat Room". New York Magazine. Retrieved October 13, 2018.
  49. ^ Lardinois, Frederic (February 6, 2017). "And the winners of the 10th Annual Crunchies are..." TechCrunch. Retrieved October 13, 2018.
  50. ^ a b Cohn, Cindy; Gebhart, Gennie (February 14, 2018). "The Revolution and Slack". Electronic Frontier Foundation. Retrieved October 13, 2018.
  51. ^ Reitman, Rainey (July 10, 2017). "Who Has Your Back? Government Data Requests 2017". Electronic Frontier Foundation. Retrieved October 13, 2018.
  52. ^ Silverman, Jacob (December 2016). "Big Bother Is Watching". The Baffler. No. 33. Retrieved December 11, 2018.
  53. ^ Finley, Klint (March 16, 2016). "Open sourcers race to build better versions of Slack". Wired. Retrieved October 13, 2018.
  54. ^ Peterson, Becky (July 26, 2017). "Uber didn't like Silicon Valley's biggest chat apps - so it was forced to make its own". Business Insider. Retrieved October 13, 2018.
  55. ^ "We Tried Slack and Deeply Regretted it". June 21, 2015. Retrieved February 10, 2019.
  56. ^ "Slacks new policy lets bosses read employees dms without consent". March 22, 2018. Retrieved February 10, 2019.
  57. ^ "Slacks new policy lets bosses read employees dms without consent". March 22, 2018. Retrieved February 10, 2019.
  58. ^ "Slack it's not the drug, it's the user". February 23, 2016. Retrieved February 10, 2019.
  59. ^ "slack productivity". October 23, 2018. Retrieved February 10, 2019.
  60. ^ "why slack is bad for teamwork and why you should never try it". July 21, 2018. Retrieved February 10, 2019.
  61. ^ Finnegan, Matthew (June 5, 2018). "Microsoft Teams: Its features, how it compares to Slack and other rivals". Computerworld. Retrieved October 13, 2018.
  62. ^ "Google's Slack alternative is available starting today". Engadget. February 28, 2018. Retrieved October 13, 2018.
  63. ^ "Yahoo Together will be discontinued". Yahoo. Retrieved July 4, 2019.

External linksEdit