I have been taking a look and I think your system is exactly what I need.
My only concern is that a user is required for most actions like posting to facebook and posting scores to a leaderboard.
Do you have to create a user before using these functions, objiously for leaderboard you are going to want to tie up a user to a score but do you have to use the User Service to create a user first?
I have an app that is very small and I think that it may be a bit of overkill to get the user to enter a username and have it check if it exists etc, jsut to post scores to leaderboard and facebook.
Any help would be much apreciated.