Have you considered Mozilla Persona? https://voo.st https://voo.st is pretty successfully using Facebook and Persona together for auth, at least the last I hear

Have you considered Mozilla Persona? https://voo.st https://voo.st is pretty successfully using Facebook and Persona together for auth, at least the last I heard from them. You should be able to get it integrated in an hour or so, and I'd be happy to double-check your code (or submit a pull request myself) if you'd like.