First, there was an online multiplayer game, Glitch. It was made by Steward Butterfield and his Tiny Speck company. The game had an internal chat system. It was divided into tabs that worked more like chat rooms. This feature became very popular among the players, but the game itself failed.
Butterfield's team noticed the popularity of the chat system. They decided to make it a separate project - a team communication platform. The tool was originally called Linefeed. They used it as the foundation to build Slack on it. Then, they asked their colleagues and friends from other companies to test the app.
Problems appeared when more users joined Slack, so they needed to tweak it a lot. They announced its public release on Twitter. It was their beta, but they didn't call it like that officially because people may have lost interest.
Thanks to Butterfield's network, 8,000 users joined Slack on day one. Butterfield told his friends and colleagues about his new platform. The word-of-mouth power granted Slack's initial success.