Facebook now supports XMPP. Yay.
Using Facebook Chat in bitlbee is pretty straight forward (I am running 1.2.4, older versions of bitlbee require a bit more tweaking, consult the manual).
For Facebook Chat through XMPP to work you need to set a Facebook username.
In the &bitlbee channel
account add jabber <username>@chat.facebook.com> <Facebook password> account on
Edit (04-03-2011): As some has pointed out, you need to input your username in lowercase.
Facebook users are listed by their Facebook user id (uXXXXXXX) so you have to rename them manually, unless you are using irssi.
Download this script to
~/.irssi/scripts and load it
/script load bitlbee_rename
(Note: This script only renames your Facebook contacts – The original version which renames all contact but does not handle international characters really well is available here)
Now each of your contacts will be renamed based on their real name in their vCard.
Remember to symlink to script to
~/.irssi/scripts/autorun if you wish to auto-load it.