Thanks for your comment!

The reason we decided to use FB only auth is that we felt that in order to provide the same feature set with FB and without it would require quite a lot of duplication on our site. And the more social features we add - the more duplication it requires.

For example, for a plan you can specify "friends only" visibility - and for this to work we need to support friends lists in our app. We also use FB to send SMS reminders (if allowed by a user) - and that's not the easiest and cheapest thing to implement from scratch.