1. From the Messages application menu, choose Uninstall Messages Beta….
2. A dialog will appear, asking you to confirm uninstallation of Messages Beta and reinstallation of iChat. Verify that you wish to do this.
3. Restart your Mac when prompted.
Facebook will now connect via the officially-sanctioned mechanism (Facebook page on the API). You no longer have to provide Adium with your username and password; when you first log in, you will be prompted to authenticate via a facebook.com window. Existing Facebook Chat accounts are upgraded automatically.