Hey Ben,
If they sign in with their Facebook account, there will not be any notification on Facebook to ask them to register on your site because by clicking on the sign in with Facebook, they are already creating an account on your site
This allows EasySocial to import some of their profile details from Facebook over (Facebook does not allow importing all of the user's data though)
I believe if you are advertising your site on Facebook, it is perfectly fine so long you don't try to compete in Facebook in that ad
You could always go around with the idea of joining a niche network to collaborate or share ideas