This was a 3-day hack to test a mobile version of our iPhone app (Tiny Review). It worked out pretty well; next week we'll convert our existing site tinyreviewapp.com into the same hmtl5 mobile format.
Yes, asking for FB permissions twice was tripping over our users however. What seems to work right now: login with Twitter or username/password, and add FB auth later when a user wants to share. This apparently doesn't feel the same as allowing FB on signup. Makes sense?
Not much - you can always login by granting FB permissions again, and you'll return to the same account. FB just provides you with a facebook id that you use to lookup the user in the db.
Initially we had a 2-step process: ask for email permissions only during signup, and for post permissions later when you actually wanted to share something. Unfortunately this confused our users even more - they didn't understand why they had to FB-auth twice. I guess that many don't actually read the list of permissions (although a short list is better than a long list).