I've made a game in unity3d for iOS and android. We have our own server to manage high scores and stuff like that. We'd also like to have the possibility post high scores to facebook, and also do things like this.. If you and your friend are have both posted a score for our game to facebook and you post a better score then you can send them a notification. I'm reading around about this now, but I'm wondering whats the normal way people do this? Possible ways..
Use the unity facebook SDK Looks like it would work but there are different versions for iOS and android.
Call the facebook graph API directly from our server. This would unify the iOS and android versions and also it makes sense as our server holds / deals with all the highscore info. I can just imagine difficulties with logging in / authentication