public function fb($title = null, $site = null, $url = null, $description = null, $type = null, $app = null)
    {
        if (!is_null($title)) {
            $this->property('og:title', $title);
        }

        if (!is_null($site)) {
            $this->property('og:site_name', $site);
        }

        if (!is_null($url)) {
            $this->property('og:url', $url);
        }

        if (!is_null($description)) {
            $this->property('og:description', $description);
        }

        if (!is_null($url)) {
            $this->property('og:type', $type);
        }

        if (!is_null($app)) {
            $this->property('fb:app_id', $app);
        }
    }