add_action( 'wp_ajax_fb_custom', 'fb_customizer_ajax' );
function fb_customizer_ajax() {

    if ( ! wp_verify_nonce( $_POST[ 'ajaxnonce' ], 'fb_customize_ajax_nonce' ) ) {
        die( -1 );
    }

    $current_url = $_POST[ 'current_url' ];
    $current_id  = url_to_postid( $current_url );
    echo $current_id . '<br>' . $current_url;
    die();
}