[lucasar]

You have no idea what you're talking about. Apps use the Facebook library because a good portion of end-users want to be able to login with a Facebook button --or Google, or whatever that doesn't require them to create a user/password account. It's just that simple.

[surferbayarea]

I have been witness to such business partnerships to embed SDKs to siphon telemetry/other data. The world is not as simple as you think...

[MCKapur]

well, this would be extremely believable

[ratww]

If you only need login, then you can have the same functionality without the Facebook SDK or Google SDK by using OAuth.