[presto10]

iChat: Add new account in Preferences, select type Jabber, enter username "<username>@chat.facebook.com" (where username is selected in Facebook settings, and you need to logout from Facebook after that change). For password use Facebook password.

[jackowayed]

Same idea for pidgin:

New account, select XMPP as protocol. Username is your username (which is your slug if you have a custom url. So I'm facebook.com/daniel.jackoway, my username is daniel.jackoway). Domain is chat.facebook.com. Password is your password. Only gotcha is that you then have to go to the "Advanced" tab and uncheck "Require SSL/TLS".

[stilist]

I’m chatting with someone who hasn’t set a username, and it shows in iChat as [e-mail]. Might work on both sides.

[hachiya]

BitlBee:

  account add jabber [e-mail] FACEBOOK_PASS
  account list
  account add 2 (or whatever number the above list shows is Facebook)
  # you'll need to do a /whois on your friends and rename them: 
  rename u393222111 John
  rename u134322433 Bob

[yurifury]

Thank you very much.

The renaming is fairly annoying, can anyone think of a way to auto-name users?

[hachiya]

A script to do /whois the first time, and rename the nick to the shown username would work.

Here's one: http://a.ngus.net/bitlbee_rename.pl

[MikeCapone]

Thank you, it worked.

At first I didn't log out after setting my username and that made it hang. That step is important.