/**
  * Returns the URL to send the user in order to log out of Facebook.
  * @return string|bool The url to log out
  */
 public function end()
 {
     if (is_null($this->fbsession)) {
         return false;
     }
     return $this->fbsession->getFacebookLoginHelperInstance()->getLogoutUrl(FacebookSession::newAppSession(), "/");
 }