The kind folks over at Facebook have now added a Jabber interface to Facebook Chat. To use the Jabber support in Adium you first need to create a Facebook username:

1. Head over to facebook.com and login.
2. Then click on the Account dropdown menu on the top right of the screen and choose Account Settings.
3. You will then see an option to change your username.
4. Choose a username and save it (this means your friends can view your profile using www.facebook.com/username (just like Twitter).

Now that you have your username, open up Adium and click:

File -> Add Account -> Jabber.

Then enter:

{yourfacebookname}@chat.facebook.com

for the Jabber ID and your regular facebook password. Click OK and you should now be connected to Facebook chat.