Though Slack officially launched just over a year ago, the app has seen truly remarkable growth in that time. At launch, in February of 2014, Slack had around 15,000 daily users. [2] By August of 2014, Slack’s daily active users had grown to 171,000, and that number had swelled to 285,000 by November. [1] By February of 2015, the app boasted over 500,000 daily active users, with “tens of thousands” of new users added week over week, representing a thirty-threefold growth in just a year. Not only that, but Slack users are collectively sending 300 million messages every month, and users are actively engaged with the app for more than two hours every day. [2]

https://growthhackers.com/companies/slack-fastest-growing-b2b-saas-business-ever/