Chat with Facebook friends within iChat | E-mail and Internet | Mac OS X Hints | Macworld: "Alternatively, you can just follow the instructions right here:
In iChat, open Preferences, and go to the Accounts tab. Click the plus sign at the bottom of the window to add a new account.
Select Jabber Account from the Account Type drop-down menu.
Enter your_username@chat.facebook.com in the Account Name field, and enter your Facebook password in the Password field. Replace your_username with your Facebook username, obviously.
Click the triangle next to Server Options to reveal those settings. Enter chat.facebook.com in the Server field, and 5222 in the Port field. Make sure the Use SSL box is not checked.
Click Done.
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