protected function _get_thumbnail( $content ) {
		preg_match( '/<meta +?property=["\']og:image["\'][^\/>]*? content=["\']([^"\']+?)["\'].*?\/?>/si', $content, $reg );
		if ( empty( $reg[1] ) ) {
			return;
		}

		$thumbnail = $reg[1];
		$thumbnail = $this->_relative_path_to_url( $thumbnail, $content );

		if ( is_ssl() ) {
			$thumbnail = preg_replace( '|^http:|', 'https:', $thumbnail );
		}

		$requester = new Requester( $thumbnail );
		$response  = $requester->request( $thumbnail );
		if ( is_wp_error( $response ) ) {
			return;
		}

		$status_code = $requester->get_status_code();
		if ( 200 != $status_code && 304 != $status_code ) {
			return;
		}

		return $thumbnail;
	}