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I trust facebook.com (sort of), so I put info on there such as my likes. I don't trust just any old site I stumble across on the internet. So there's no way that I'm going to register for your app with my facebook account.

However, after some use, I may find that I do trust your app. Then I might be ready to click "connect with facebook". (Although that's very unlikely, as I haven't even done this with Netflix, which I've had for years).

So the best you can do is:

  1. Provide your users with a way of connecting facebook after they have signed up, and let them know about this during registration (show "You can always connect to facebook later" on the sign-up page).

  2. Occasionally remind users of the benefits of connecting facebook in a non-obtrusive way. (Occasionally in the content of some of your pages, display a message like "Connect with facebook and see what your friends are [watching/reading/whatever it is people do on your app]! We will never post anything on your wall without asking." with a Connect Facebook button.

  3. Advertise on facebook to get potential new users to come to your site from facebook.

  4. Accept that some (probably most) people just won't use facebook on your app, and find another way to collect information about their preferences (like showing movies or books and allowing the user to check the ones that he or she prefers, or perhaps rate them).

  5. Make sure you have a Privacy Policy that is clear, easy to understand, and easy to access about exactly what information you get from facebook and how you will use that information.