Time was when Google and Facebook both used XMPP (I think with official cross network support in their apps?). I also recall MSN Messenger being able to talk to Yahoo Messenger, though I'm not sure if they used XMPP for that.
Google talk was XMPP-federated for a while (but never upgraded to good practice such as using basic encryption...), and while you could connect to Messenger via an XMPP client, they never enabled federation.