Use Facebook Chat in iChat
It is now possible to connect to Facebook Chat through a desktop client using the Jabber protocol. This means that you can chat with your Facebook friends without having to log into Facebook. The instructions here are for iChat, but it works with loads of other messaging applications too, such as Adium and Pidgin.
Start by opening up iChat, then go to the iChat menu and select "Preferences." In the Accounts tab, click the plus button (+) and enter the following settings:
Account Type: Jabber
Account Name: [username]@chat.facebook.com
Password: Your Facebook password
Uncheck "Use SSL" if it is checked.
Your Facebook username is the name that appears in the address bar when you go to your Facebook profile page. If you can't figure it out, go to this page on the Facebook site to find it.
Finally, click Done, and a new buddy list should appear with all your Facebook friends. It should also sort them into the correct groups if you have set them up in Facebook.