And I genuinely have no idea i) what you can do to my timeline or ii) what information you get about me from FB.
1. I don't know at the time what you want to use my information for, especially in regards to showing up on my or, more importantly, my friends' news feeds.
2. I have to manage what information I share on Facebook itself rather than on the app or website.
It's not just you. I wouldn't create a Medium account yesterday because Ev Matthews wants me to sign in using Twitter. I hate Twitter, why would you want to restrict yourself that way?
I can log in just fine, I just don't want to and your constant pestering is only making things worse. I'm also against the concept of single sign on. Users constantly forget their passwords and have their accounts compromised.
The last thing you need is to lose access to things you've paid for because someone hijacked your Facebook account.
Foursquare shows a nice way to do it: https://foursquare.com/signup/ though hopefully you don't need as much data as them.
In my experience most people will still choose FB login if you make it the prominent choice, and the email sign up will capture us whiners.
Even when I did have a facebook account I wouldn't have been comfortable using it to log in to other services, because you never know what information Facebook is going to mine out of anything you give it access to.
Not everybody thinks Facebook is creepy but their brains do, I don't think it's a good idea to stir those creepy feelings in your user's subconscious and risk having them projected at you, if you can avoid it.