Hello. In looking at the API/products facebook offers that URL api.facebook.com/restserver.php was not something I could find. Can you point to the Facebook API documentation of the particular endpoint you wish to access?
Also, the programmableweb entry you provided shows that authentication in the form of oauth, apikey, username/password is required. But, it sounds like those weren't set. Might be better to just implement a process to listen to/interact with the Facebook API.
Thanks
Joe