I've tried it before on a previous project of mine, and the results were pretty bad. Many users walked away because it wasn't obvious why we needed access to an

I've tried it before on a previous project of mine, and the results were pretty bad. Many users walked away because it wasn't obvious why we needed access to any of their Facebook information--we weren't asking for any permissions until much later in the relationship, but users still balked. There weren't any great ways to notify a user and it was unnecessarily hard to allow users to interact with people for whom they new an email address but weren't friends on Facebook. I think your best bet is to encourage users to create an account on your site/app directly (perhaps with a Facebook/Google option), and then later link it to Facebook once they understand the benefit to them of doing so.