For all of you who have noticed a problem with Facebook accounts, where user names will disappear and will be replaced by numbers:
Facebook seem to be preparing an upgrade of their XMPP service that should fix the issue. Current ETA is about two weeks according to them.
Until this happens you can use the new version at beta.chat.facebook.com . In other words you just create a new XMPP account using <your.fb.username>@beta.chat.facebook.com as the user ID. Hope this helps.