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.
Futures/Promises are an interesting technique to do control flow. IMO, there are certain nuances like losing distinction between async/sync calls that work against the technique. I wrote an article about it at: http://javascriptturnsmeon.com/93769736