You can upload this simple PHP script to a public server on the Internet and receive there callbacks from webservices like Facebook's Graph API and have it redirect the request to localhost or any other non-public URI.

ws_callback_redirector.php

<?php

define('APP_PREFIX', '/path/to/subfolder/if/any/');
define('REDIRECT_TARGET', 'http://localhost:8000/');

if (array_key_exists('redirect_target', $_REQUEST)) {
    $redirect_base = $_REQUEST['redirect_target'];
} else {
    $redirect_base = REDIRECT_TARGET;
}

$url = $_SERVER['REQUEST_URI'];
$url = str_replace(APP_PREFIX, $redirect_base, $url);
header("Location: $url");

?>