Facebook and Google? They use IRC internally for technical reasons. IRC doesn't fall over. Googlers internally revolted when they were told to use Hangouts internally.

XMPP works great and I've used it at several employers. There's no technical reasons to avoid it; it really is all about lock-in and corporate planning.

I've watched Ralph struggle for years against corporate asshats. It's a real tragedy that all these companies don't participate in open standards. We shouldn't have ever expected good things from Slack.