I am co-founder of Sweetch. We develop an iPhone app to help drivers find parking on-street instantly in San Francisco. 10% of our users did not manage to login to our app because of FBLogin and I just figured out why. We used to ask FB API for the email address of our users. It turned out that, in some cases, the new FB API doesn't return this information even when the user give you the permission to access it. The reason is that some of the users have set their Facebook email address (name@facebook.com) or their phone number as main email address in Facebook. Many apps (including Airbnb) have this issue but don't even know about it.
If you want me to help you to debug, leave comments on this post ;)