I don't think it's a good idea to register a .NET user when someone logs in with Facebook Connect. Think of then having to manage that user, say if someone changes their Facebook name or other public information. I think your much better off changing the
way your application behaves if a Facebook user signs in.
You can use the FBUID as the asp.net auth token:
Facebook.Session.ConnectSession session = new Facebook.Session.ConnectSession(APIKey, APISecret);
Facebook.Rest.Api api = new Facebook.Rest.Api(session);
api.Session.SessionKey = ConnectAuthentication.SessionKey;
api.Session.UserId = ConnectAuthentication.UserID;
Facebook.Schema.user user = api.Users.GetInfo();
// You can store facebook data in a local table now by making API calls. For instance you migth want to store FBUID and name so you dont have to do API calls later in the session.
The only downside is if you have User.Identity being shown on the page anywhere you will have to go get the FB name, whether that means you make an API call, stored data, or some sort of session variable.