BigLouis87 on "Facebook federated login"

Thanks Stephan, I studied your script and understood that its core is the static function federated_logon::credentials.
Using the facebook php sdk I solved the fb auth part, so I finally have an array with user data; my idea was passing this data to the federated logon:
if (fb user logged in) federated_logon::credentials('fb:'.$user_profile['id'], NULL, $user_profile['name'], $redirect);
I made this a plugin; it partially worked, but even if it create the user and log in (if I go to /admin on top I can see "fb:xxxxxxxxx" as the username), the $_zp_current_admin_obj object is NULL so other functions that use this object don't recognize the user as logged in.
Is this a normal behaviour using federated_logon::credentials? How can I fix this?
I load this function calling it in the plugin with zp_register_filter('theme_head', 'FBloginHeader');, is this correct?
Thanks