So true, XMPP is fantastic. I also use it for SMS and signal (I wrote some simple bridges for Prosody). We use it for work too. It's great to have it on all my devices. Group chats (MUC) also work well. The modularity is where it really shines. Conversations on Android keeps a long running TCP session open, it works better than push based solutions and is just as light on battery somehow.

I mean, it still exists, but approximately no-one uses it. There was a time when you could talk to people on Google and Facebook's chat systems with it.