We do make extensive use of Facebook messages (many people, including me, make good use of Facebook Messenger on mobile and/or desktop) and of Facebook groups.

We do make extensive use of Facebook messages (many people, including me, make good use of Facebook Messenger on mobile and/or desktop) and of Facebook groups. IRC's ability to quickly create temporary groups, temporary membership (essentially muting discussion by leaving, or peeking by joining), bot frameworks, and multiple clients are potential reasons why IRC might be preferable for the sort of things it is used for, I guess.