1. App-scoped User IDs
Facebook will begin to issue app-scoped user IDs when people first log into an instance of your app coded against v2.0 of the API. With app-scoped IDs, the ID for the same user will be different between apps.
No matter what version they originally used to sign up for your app, the ID will remain the same for people who have already logged into your app. This change is backwards-compatible for anyone who has logged into your app at any point in the past.