Follow the steps carefully to chat your facebook friends in ichat.

STEP 1. From your Applications folder you have to launch iChat.

STEP 2. From the iChat menu, click on Preferences.

STEP 3. Now you have to add a new account. For this simply select Accounts tab then click “+” button which is at the bottom left of the window.

STEP 4. Now simply Type dropdown from the Account and then click on Jabber.

STEP 5. Now you have to enter your username and password.

STEP 6. Now expand the Server Options, then set the Server to chat.facebook.com and the Port to 5222.

STEP 7. Now simply click on Done button. Now a new account will be appeared in the list on the left side of the window.

STEP 8. Now simply sign in and you can use iChat to send message to your Facebook friends.

Source: SolidBlogger