[rmrfrmrf]

Facebook offers libraries for iOS (and Android) developers to add features Facebook login, but which include other features like native app analytics and drop-in widgets that serve Facebook ads.

If it's not obvious, these Facebook libraries are trojan malware, full stop. And, even though iOS apps are sandboxed, if 50% of a user's apps have the same Facebook library baked into them, it's not hard for Facebook to link all that data together to determine usage patterns across apps.

The reason why they're fighting with Apple now is that it used to be much easier for Facebook to obtain a device-unique identifier that each app could forward along to Facebook, giving them device-level usage data. Apple now sees the privacy flaws in device-unique ids, and has either started or plans to start rotating those device ids and making them unique at the app-level only, which reduces the resolution of its tracking from the device level to the network level.