$data = parse_signed_request($_REQUEST['signed_request']);
$fb_id = intval( $data["page"]["id"] );

$sites = wp_get_sites();

foreach( $sites as $site ) {

    switch_to_blog( $site['blog_id'] );

    $fb_page = get_option( 'fb_pages' );
    $fb_page_id = $fb_page['fb_page_id'];

    if ( $fb_id == $fb_page_id ) {

        $args = array(
          'numberposts' => 1,
          'post_type'   => 'tab',
        );

        $tabs = get_posts( $args );

        if ( $tabs ) {
            foreach ( $tabs as $tab) {

                wp_redirect( get_permalink( $tab->ID ), 301 );
                exit;
            }
        }
    }

    restore_current_blog();
}