It's also used for video games a lot, but between leaking your IP and allowing esports players to get DDOSed, and just generally being not a great fit, Discord is getting more popular.
http://www.theverge.com/2014/10/8/6946341/slack-has-a-seriou...
https://www.reddit.com/r/GlobalOffensive/comments/382mao/psa...
What's confusing is the second link is about Skype leaking your IP, not Slack.