The Facebook profile appears to provide options only for a restricted list of social links. How do I add a different one?
A Facebook account comes already with a Jabber ID,
email@example.com and the password is the same as your account.
You can find it in the profile; it's called "username" or something like that.
However, a few months ago when you try to use and external XMPP client, Facebook will alert you to allow such type of connection. And now you can't use any unofficial XMPP client. Facebook has blocked such access.