We’ve seen hard numbers about this exact issue. On October 8 we released the first version of the app with FBSDK 4.6 In the same update we made our login flow more prominent in the UI, so we saw a significant uptick on email signups. However, our Facebook logins went down at the same time. This effect did not occur for iOS 8 users (email and facebook logins went up together). Here is a graph. Red is email signups, blue is facebook for iOS 9 users only.
Facebook vs Email signups chart
tl;dr The latest update to the Facebook SDK killed our facebook conversion rates!
@ CanAksoy Is Facebook seeing the same effect internally? I agree that our interests are aligned here, what trend has moving everyone to Safari had on your conversion rates globally?