Correct me if I'm wrong, but isn't the point of have a "Login with FaceBook" option to have extra security so that if the server is hacked your password isn't stolen? That wouldn't work with RoundCube because RoundCube needs the email account password to log in to the mail server. So if you were to use a "Login with FaceBook" system there would have to be a database of usernames and password for RoundCube to pull from there for making the "extra security" completely useless.
