Actually, searching for this stuff made me sad :(
There are blog posts from Facebook still up about how they are using XMPP [2]. There are still notes out there on setting it up in Pidgin [3].
The internet has become a walled-off mess, and it makes me depressed to see such a powerful and wonderful technology abused like this.
[1] https://stackoverflow.com/questions/5052780/integrating-face...
[2] https://developers.facebook.com/blog/post/110/
[3] https://m.facebook.com/notes/socialblogr/how-to-setup-facebo...
PS: looking through the other posts on the fb developer blog (just changing the number in the url) is interesting.