Facebook profile pic used as avatar

Question

I'm trying to add an avatar option for my blog and would like to avoid using Gravatar or custom avatars so I wish to use Facebook profile pic instead of an avatar. I know that there are plugins for that but they're all a bit cumbersome.

I tried Incarnate but the issue with it is that there is no authentication so anyone can use any publicly available username thus misrepresenting themselves. Also, if a person doesn't have a publicly available FB profile, they can't use their profile pic.

To sum up, I'd like a plugin or a code so that visitors to my blog are automatically represented with their FB profile pic (if they are already logged in to Facebook) without the need for them to register or log in with FB details to my blog. If a person is not logged in to FB, then a custom avatar is to be used. Please suggest a solution.