Not sure about the actual statistics, but XMPP is not just a standard for messaging between people: I think it's also intended as a backend technology for communication behind automated services. So even though there may not be many humans using XMPP as a messaging application, there may be many people who "use it" without noticing (as in, use a website, etc., which relies on XMPP somewhere).

I don't know how one could get a good estimate of this, but it is still very widely used. Whatsapp (with custom compression and federation disabled), HipChat Server, Cisco Jabber, Android push notifications, Google Cloud Print communications, etc. are all XMPP. But yes, for personal instant messaging Google and Facebook were the big players and they are no longer on it (at least as an interface to their clients)