At the first glance, I was very impressed with the design but I was unsure what the website is about. I was able to test out the app and what I got from it was that it is a slideshow with captions. Am I missing any features?
Could you give more info/insight on what your app exactly do in the end? I really don't feel comfortable using my facebook account just to test something and give feedback as I'm seeing in most of Show HN posts.
The app helps you create beautiful wedding invites. A lot of people create FB events or send emails for this scenario which I dont think is befitting for an occasion like a wedding.
As far as FB login is concerned, you will notice I ask no permissions(not even email) when you login and grant permissions through Facebook. This means that the app really can't do anything.