Just Google for 'facebook login with oauth asp.net' and you'll find plenty of resources.
You can also use Federated Authentication with an identity broker such as Auth0 to aggregate many different authentication providers. I wrote a blog about it:
I am using usual login and facebook login in my site.
Usual login requires user to register in the site. On registration I am creating a user using MemberShip class.
My question is how do I create a user if logged in through Facebook. Is an approach of creating a user using MemberShip class valid for Facebook Login.