I have just succeeded with doing exactly that. I've added Facebook to RTCOMM and it works. You can create an account, login and it will work.

I won't be releasing it until I can talk to some of the guys from Collabora first because the side effect is that the Contacts app stops showing you who is online for all of the RTCOMM protocols. This is probably because of the plugins for Haze missing something it requires.

Some additional details... when you first message a Facebook contact, their name will be shown as their Facebook profile number. After a minute or so, it will be replaced with their profile's real name. Also, even though the Facebook icon appears to be in the right place, the UI doesn't pick it up.

Facebook updates (when people comment on something you've done, for example) are properly sent and received however RTCOMM does not actually show this to you. I know it's there because the incoming messages show up in the syslog.

Once I can get a hold of the Collabora guys and we can get some of the kinks worked out, I'll release... or maybe Collabora will release a package for adding Facebook.