Comment by seba_dos1

The same could be said about various XMPP transports that I've used back in the day with Google Talk to access all sorts of IM networks. Facebook was just running one on their servers rather than you running it on yours.

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).