Practically all internal communication is done with Facebook, because why not? It's the lowest common denominator. Messaging is done through Facebook messenger, teams are private groups, etc.
Except one group has resisted this: the sysadmins.
Sysadmins need to be able to communicate at all times, /especially/ if things aren't working. And it turns out that there's no way to make sure things like Facebook messenger always work—especially during outages. They're just too damn complicated.
So, they came to a compromise. The rest of the company will move to Facebook, and the sysadmins will stay on IRC, the communication platform they know they can rely on.
When Facebook's on fire and the company is panicking because they can't communicate, you can be sure of one thing: the sysadmins are chatting away on IRC about how to get it working again.