They actually are changing the Photos privacy defaults in iOS 11: https://developer.apple.com/videos/play/wwdc2017/505/
Apps won’t need to ask for permission to your entire library - the Photo picker will just appear, and only user selected photos will be available to the app.
On the iOS 11 GM at the moment, Facebook still seems to need access to my entire library. So perhaps it is only for apps that link against iOS 11. Guess we’ll see soon.