Facebook had an XMPP interface that was buggy and sucked ass, so you'd have to check the web interface to see if messages actually got delivered. Facebook also tried @facebook.com e-mail address for all users at one point, which they also killed.

FB's XMPP never federated. Google's did, but they silently killed federation around .. 2013 or 2014 I think. Pissed a lot of people off.