Login to Facebook chat from iChat

1. Go to your profile on Facebook and copy your username, which looks like this: http://www.facebook.com/your_username (in bold).
2. In iChat -> Preferences -> Accounts, click the plus (+) to add a new account.
3. Use in these settings:

Account Type: Jabber
Account Name: your_username@chat.facebook.com
Password: (your Facebook password)
Server: chat.facebook.com
Port: 5222

You're welcome.