Matrix is quite popular in Europe. German police and army uses it in their private instances...

There are a ton of programming channels that are very active: Rust and NixOS are the ones I follow.

I think the best implementation so far is Beeper, where you have all the bridges to different chats together with Matrix. I run it so I use the same app for Signal, Telegram, Whatsapp and LinkedIn. You can set it up by yourself too, but Beeper makes it very easy. They open to public later this month.

https://www.beeper.com/