Slack spread through businesses like wildfire, initially in the tech and media sectors, but now much more widely. At its public launch in February 2014, it had 17,000 users. As of April 1st, 2016, that number had rocketed to 2.7 million daily active users.

Though it has plenty of competitors, Slack claims to be the 'fastest growing business application in history'.

Slack users I know, including me, love many things about the service. As the company likes to brag, it's fast, it's transparent, and it's great for brainstorming.

Walt Mossberg Quotes