And it's not just the Facebook apps. The Facebook SDK is widely embedded into many other apps Facebook SDK (it's the most popular SDK on iOS for example, according to data published from Cocoapods - https://gist.github.com/ryanolsonk/e33bf9e89677da9fe8ce ). It's so popular because Facebook is basically the most effective network to run so called "App Install Ads" and embedding their SDK providers marketers essential data for tuning their targeting and ad spend.

The SDK gives Facebook access to a ton of additional usage data which they can link together with anything they already have on your from their apps. It also _potentially_ gives them the ability to exploit any permissions given to the host app which embedded the SDK. Now they publish the source of the SDK openly ( https://github.com/facebook/facebook-ios-sdk ) but it certainly would harm anyone if there was more independent scrutiny given to analysing the source and whether there's anything else we're not seeing that go into the final SDK downloads.