Facebook got a new version of its login system for the third-party apps that lets you log in anonymously so you can use facebook apps without providing your personal data. The new Anonymous login also gives you the total granular control over what data types you give to an app.
Even with an anonymous identifier, user’s experience between apps can still be synced, but the app will not get any personal information from the user. This will work because Facebook still use unique identifier for you every time user log in anonymously. Thats way, even if an app does not know your name, then it can save your content, high scores or there data and sync it across the devices.
It is really important to understand that when Facebook says “anonymous”, what it means is that “you do not need to give third-party developers your name or data before you use Facebook’s apps”. However, Facebook will still going to gather data about what apps you are using “anonymously” that it will use to personalize your other Facebook experience or potentially targets you with the ads.