Under 'App review' there's an option to make your app public. Right now it says 'No' and 'Your app is in development and unavailable to the public'. What happens if I switch it to yes though. Will my friends be able to see the app i'm working on (I don't want that). So how do I keep my app development private, but at the same time be able to use it's features. Because as I'm aware, I have to turn 'Make app public' to 'on' in order to utilize Facebook login for my app.
No your friends wont be able to see the app you are working on. Making your APP public is for your clients to use it's features and not just the developer.