|Original author(s)||Stewart Butterfield, Eric Costello, Cal Henderson, and Serguei Mourachov|
|Initial release||August 2013|
|Operating system||Microsoft Windows, macOS, Linux, iOS, Android, Windows Phone|
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.
Slack offers many IRC-like features, including persistent chat rooms (channels) organized by topic, private groups, and direct messaging. 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.
Slack's free plan allows only the 10,000 most recent messages to be viewed and searched.
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.
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. 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, and Zapier. In December 2015, Slack launched their software application ("app") directory, consisting of over 150 integrations that users can install.
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.
Slack provides an application programming interface (API) for users to create applications and automate processes, such as sending automatic notifications based on human input, sending alerts on specified conditions, and automatically creating internal support tickets. Slack's API has been noted for its compatibility with many types of applications, frameworks, and services.
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. It was featured on the home screen of the Apple Watch in a 2015 promotional video. Slack has been made to run on a Super Nintendo Entertainment System via Satellaview.
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)". That specific limit no longer applied by January 2017.
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.
8,000 customers signed up for the service within 24 hours of its launch in August 2013. 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. By April 2015, those numbers had grown to 200,000 paid subscribers and a total of 750,000 daily active users. Late in 2015, Slack passed more than a million daily active users. As of May 2018[update], Slack had over 8 million daily users, 3 million of whom had paid accounts. 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.
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. 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".
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". 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, 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.
- Internet Relay Chat (IRC)
- Matrix (client: Riot)
- Keybase (open-source client with proprietary server)
Other alternatives having a similar business model to Slack are Flock, Microsoft Teams, Hangout Chats, Symphony Communication, Trillian, Workplace by Facebook, Discord, Glip, Cisco Webex Teams, and Yahoo Together (now defunct).
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