One of the hidden gems of Facebook Chat is that it integrates quite nicely into iChat. Here’s how to set that up.
First go to iChat >> Preferences
Click on the plus in the lower left corner to add another account.
In the account setup window, select Jabber for the Account Type.
In the Account Name field, enter your Facebook screen name @chat.facebook.com. Then enter your Facebook password.
Click the arrow next to Server Options to show the Server and Port fields. Enter chat.facebook.com for the Server, and 5222 for the Port. Make sure to have Use SSL unchecked.
Click Done and iChat will automatically sign you in and display your Facebook contacts. By default, it will show up as a separate group from your other contacts. If you want one single list instead, click View in the menubar and uncheck Use Groups.
Now you’ll have your Facebook friends in iChat along with the rest of your buddies.