Internal teams at IBM reportedly started using the chat app as far back as 2014, and this has grown over time.

Slack CEO Stewart Butterfield told Business Insider: "Going wall to wall in IBM -- it's basically the maximum scale that there is, so we now know that Slack will work for literally the largest organisations in the world." 

While this new rollout makes IBM Slack's biggest customer to date, it has been the company's biggest customer for years according to Slack. "IBM has been Slack's largest customer for several years and has expanded its usage of Slack over that time", reveals an SEC filing from Slack, which appears to downplay the news.