After 3 PM, most of the core features became operational, except for push notifications, email, and third-party integrations including Google Calendar and Outlook Calendar. From 10 AM ET until 3 PM ET users could not log-in, send or receive messages, place or answer calls, and use Slack connections. On January 4, 2021, Slack suffered a large outage which lasted several hours. On January 1, 2021, Slack and Salesforce announced an agreement for Salesforce to acquire Slack for a value of approximately $27.7 billion.

'Searchable Log of All Conversation and Knowledge' is a backronym for 'Slack'. 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. In June 2019, Slack went public through a direct public offering to reach a market value of US$19.5 billion. In response to the attacks, Slack added two-factor authentication to its service.

In March 2015, Slack announced it had been hacked over four days in February 2015, and that some data associated with user accounts had been compromised, including email addresses, usernames, hashed passwords, phone numbers, and Skype IDs. Slack began as an internal tool for Stewart Butterfield's company Tiny Speck during the development of Glitch, an online game.