How to use Facebook Chat with iChat

First, find your Facebook username by going to http://www.facebook.com/your_user/. Next, Open iChat, then select iChat » Preferences and click on the Accounts tab.

Click on the + (plus) sign to add a new account, with these settings:

Account Type is Jabber Account

Account name is your_user@chat.facebook.com, and enter your password

Click the drop-down arrow to reveal Server options. Enter chat.facebook.com as the server name.

Enter 5222 as the port and click Done.

Click Done again, and you are good to go.

via How to use Facebook Chat with iChat – Mac OS X Hints.