As of about an hour and a half ago, Facebook Chat has launched support for Jabber/XMPP.

Reference Facebook Blog post : http://blog.facebook.com/blog.php?post=297991732130

To get Facebook Chat running on your n900 without ANY extra programs, using only the built in features of a stock n900, follow these instructions:

1. Go to the Settings Menu
2. Go to VoIP and IM accounts
3. Press New
4. Select Jabber
5. Press Register New Account (necessary even if u already have a facebook profile, note that if you don't have a facebook profile i don't know if you can do this)
6. In the 'User Name' field, type in '<facebook-username>@chat.facebook.com' substituting your facebook vanity profile name for <facebook-username> and obviously omitting the single quotes.
7. In the 'Password' field enter your current facebook password, then enter it again in the Verify field
8. Press Advanced Settings
9. In the 'Connect to server' field, type 'chat.facebook.com'
10. Ensure that Port is set to '5222'
11. STUN settings are unneeded, leave default
12. After finishing registration, login with username: <facebook-username>@chat.facebook.com' and your facebook password.

Please note that this will pollute your Address Book with all your facebook contacts so be prepared to clean up