Hi,
The Jabber Account you have (Possibly GoogleTalk ID or Facebook or other Jabber ID) will "read" the Buddy list you have from the Jabber server the Buddies are on (in this case Facebook).
The Facebook server will be the place that is sending Numbers in place of IDs
The iChat/Messages software is the bit that joins that up with your Address Book/Contact app and lists the persons "Real Name" in the Buddy list.
If a Buddy is not in your Address Book (Contacts) then the Buddy list should show the Jabber ID (or AIM Screen Name or Yahoo ID).
Technically this is a privacy issue as the numbers are more than likely the actual registration number that was generated when they registered and not designed to be public.
People should be complaining to Facebook.
8:11 pm Friday; July 26, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously