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 email@example.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.