Integration of Facebook and asp.net membership

Question

I want to allow users to sign up to my website using Facebook. I already use the JS SDK on the client side for authentication and authorization and the C# sdk on the server side

Answer 1

1 option: You should generate a password and send it by email to the user, for example:

Welcome to my superwebsite.com, login: foo@foo.com password:dhyfd46

Then if you see that the password is the original (generated) ask the user to change it for another one.

2nd option: you will never use a password, for you an account is a tuple (facebookID,email) (you will get it from the facebook Api) and if it will match this (the user will click om 'login with facebook', you will be able to create the user session

After the Edit:

Why not using the tuple (userId,facebookId) and if you have the fb Id you know the userId?