Facebook never had "first-class integration". It was just a client bridge - you could login into Facebook Chat using your XMPP client, but it was a completely separate network, unlike Google Talk which was an actual federating XMPP server.
In any case, it contributed significantly to XMPP's reach and utility, and it's gone now.
Ultimately they just briefly used XMPP to not have to implement their own desktop client for their closed proprietary network. It had nothing to do with network reach, unlike Google Talk which did actually bring XMPP to the masses for a while (and then took it away).