(And my point regarding support on Google/Facebook was that their users could chat with me over XMPP without having to leave their familiar sites, sign up for anything new, or do anything else special. That put it in easy reach of the masses.)
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).