public function getFacebook()
 {
     if (!$this->conf) {
         $this->conf = $this->getConf();
     }
     if (!static::$facebook_instance) {
         if (!empty($this->conf->FacebookAppId) && !empty($this->conf->FacebookAppSecret)) {
             // init fb
             static::$facebook_instance = new Facebook(array('app_id' => $this->conf->FacebookAppId, 'app_secret' => $this->conf->FacebookAppSecret));
             // get access token
             $token = (string) SocialHelper::fb_access_token();
             // set token
             static::$facebook_instance->setDefaultAccessToken($token);
         }
     }
     return static::$facebook_instance;
 }