Cranberrygame's FB plugin has an expression to retrieve the users facebook e-mail so I thought that would be possible. In any case, if e-mail is not feasible, FB User ID works too. I basically just want to be able to display highscores only from FB friends. Furthermore I have set up my FB developer account and users are able to log in however anything more complicated than that and both the native plugin and cranberrygame's plugin are useless. Hence I need an alternative method.
If e-mails are too hard FB User ID will suffice. Thanks for your offer, ultimately I'd like to be able to use one PHP script to enable users to log in, share and get friend details with any mobile operating system (iOS, android, W8). Are you able to do this?
Thanks for the responses guys!