Having used it in several apps, the iOS SDK is an absolute mess.

Part of this is the absurd complexity of it: there are sign-on flows for going to the Facebook app if installed, or to Safari, or to an in-app web view.

Part of this has to do with the complexity of storing cookies in WebKit. The SDK wants to redirect to the FB app if it's installed since that is pretty much guaranteed to have the user's credentials. Safari is next most likely, since it will have the user's session cookie stored. Finally, the in-app web view is the fallback of last resort.

This complexity seems to be an attempt to save the user from entering text in a tiny form to sign in. I don't know that it makes an actual user's life easier, but it certainly doesn't help the developer.