Unfortunately Facebook XMPP doesn't support IQ routing between users meaning "NO JINGLE ALLOWED!".
This is bad not only for the Users, but also bad for Facebook, which could be happy right now, with VoIP support.
Let's hope they improve their services in order to fully support XMPP allowing users to experience the best of IM Freedom!
Anyway, this is an important step for XMPP Community which just got 400 Million potential users.