certainly wasn't anything intentional by facebook. just a result of the fact that they use their l.php script for blanking referrers before sending them to outside addresses.
so for their ad system they just need to check that the url's haven't been already blacklisted or use a script other than l.php for redirecting.
so for their ad system they just need to check that the url's haven't been already blacklisted or use a script other than l.php for redirecting.