One question: During the time which Facebook Messenger and Google Hangout are both using XMPP, is it possible for a Google Hangout user to send message to a Facebook Messenger user via Google Hangout client?
I forget where I read it now but if memory serves that was part of the reason why Google Talk xmpp federation was turned off because Facebook was importing Google Talk users but then not also sharing the xmpp federation so in order to stop bleeding users, Google Talk turned off their xmpp federation.
Hmm, I don't remember, but I doubt it. The point of FB Messenger using XMPP was that you could add your FB account to your XMPP client and chat with your friends from there
This is an anecdote until I can find a source.