How do I Authenticate a user using his/her FB account in C#?

<script>
 window.fbAsyncInit = function () {
              FB.init({
                  appId: 'XXXXXXXXXXXXXX',//get app id from fb developer app
                  status: true,
                  cookie: true,
                  xfbml: true,
                  oauth: true,
              });
          };
          (function (d) {
              var js, id = 'facebook-jssdk'; if (d.getElementById(id)) { return; }
              js = d.createElement('script'); js.id = id; js.async = true;
              js.src = "//connect.facebook.net/en_US/all.js";
              d.getElementsByTagName('head')[0].appendChild(js);
          }(document));

          //LOGIN FUNCTION 
          function fb_login() {

              FB.login(function (response) {
                  if (response.authResponse) {

                      console.log('Welcome!  Fetching your information.... ');
                      access_token = response.authResponse.accessToken; //get access token
                      user_id = response.authResponse.userID; //get FB UID

                      FB.api('/me', function (response) {
                          if (response.name) {
                              var userName = response.name;
                              var r = splitUsername(userName);
                              var SNfirstName = r.firstName;
                              var SNlastName = r.lastName;
                              var SNuserId = response.id;
                              var SNuserEmail = response.email;
//here you will get fb login user data shown above r.firstName 
                              var args = {
                                  "SNfirstName": SNfirstName, "SNuserId": SNuserId, "SNlastName": SNlastName, "SNuserEmail": SNuserEmail
                              }


                              $.ajax({
                                  type: "POST",
                                  url: "edRegisterPage.aspx/SetSession",
                                  data: JSON.stringify(args),
                                  contentType: "application/json; charset=utf-8",
                                  dataType: "json",
                                  success: function () {
                                      var url = "/edRegisterPage.aspx";
                                      window.location.href = url;
                                  },
                                  error: function () {
                                      alert("Fail");
                                  }
                              });
                          }
                      });
                  } else {
                      alert('Login Failed!');
                  }
              }, { scope: 'publish_stream,email' });
          }
</script>

[WebMethod(EnableSession = true)]
        public static void SetSession(string SNfirstName, string SNuserId, string SNlastName, string SNuserEmail)
        {
            HttpContext.Current.Session["SNfirstName"] = SNfirstName;
            HttpContext.Current.Session["SNuserId"] = SNuserId;
            HttpContext.Current.Session["SNlastName"] = SNlastName;
            HttpContext.Current.Session["SNuserEmail"] = SNuserEmail;
        }