I'm using the Facebook SDK v4.6.0 for iOS 9 in a simple Swift app to [add a login button](https://developers.facebook.com/docs/facebook-login/ios) to my app (I've followed the steps in [Preparing Your Apps for iOS 9](https://developers.facebook.com/docs/ios/ios9)). 

The login button always opens Safari to authenticate, and never uses the native Facebook app. Is this the expected behavior? Is it possible to use the native app (where many users are already authenticated) instead of going straight to the web login? 

I've tried testing the app on my iPhone by building and running using XCode, and with a build distributed by TestFlight. It always opens Safari to authenticate, even though I have the native app installed and configured. 

I found [this relevant question](https://stackoverflow.com/questions/23742024/how-do-i-use-facebook-login-via-facebook-app-for-ios) where the answer seems to apply only to the 3.x SDK. I've also found other questions that had references to configuring `tryFBAppAuth` or `authorizeWithFBAppAuth` that seemed to apply to edge cases on previous versions of the FB SDK. I haven't found a clear answer for 4.6.