Comment 50 for bug 542836

I had the same problem, solution is really easy ...

Just try the following instruction, and let me know if it works for you ...
--------------------------------------------
EMPATHY PROBLEM

General description:
Empathy can't log in at the facebook chat interface

Do basic check before BUGFIX!
First, control your facebook username and your password, if your your username is active, you can visit your facebook-profile in your web-browser by tipping http://www.facebook.com/#Replace this text between the slashes with your username#/

Important-Note: Your normal name in facebook is NOT the same as your facebook-username, after normal registration on facebook, the facebook-username is normally still NOT set, you have to choose one in your profile settings or by visiting the following link http://www.facebook.com/username/

BUGFIX description:
My solution, after registration of the required facebook-username, wait a few hours, it could take some time until the account is available for any connection from empathy! If it's still not working, open facebook in the webbrowser, go to your profil settings, change your facebook-password once, logout and close your webbrowser. Set the new password in empathy and try to connect again!

Now it should work fine...