MainActivity.cs

        void LoginToFacebook(bool allowCancel)
        {
            var auth = new OAuth2Authenticator(
                clientId: "395508370820207", // APP ID retrieved from Facebook created Application
                scope: "email",  // the scopes for the API
                authorizeUrl: new Uri("https://m.facebook.com/dialog/oauth/"),  // the auth URL for the service
                redirectUrl: new Uri("https://www.facebook.com/connect/login_success.html"));

            // To allow cancel of the authentication
            auth.AllowCancel = allowCancel;

            // If authorization succeeds or is canceled, .Completed will be fired.
            auth.Completed += (s, ee) =>
            {
                if (!ee.IsAuthenticated)
                {
                    // Not Authenticated
                    var builder = new AlertDialog.Builder(this);
                    builder.SetMessage("Not Authenticated");
                    builder.SetPositiveButton("Ok", (o, e) => { });
                    builder.Create().Show();
                    return;
                }

                // Now that we're logged in, make a OAuth2 request to get the user's info.
                var request = new OAuth2Request("GET", new Uri("https://graph.facebook.com/me"), null, ee.Account);
                request.GetResponseAsync().ContinueWith(t =>
                {
                    if (t.IsFaulted)
                    {
                        // Problem in authentication
                        var builder = new AlertDialog.Builder(this);
                        builder.SetTitle("Error");
                        builder.SetMessage(t.Exception.Flatten().InnerException.ToString());
                        builder.SetPositiveButton("Ok", (o, e) => { });
                        builder.Create().Show();
                    }
                    else if (!t.IsCanceled)
                    {
                        // Authentication succeeded, show the toast then go the main activity
                        var obj = JsonValue.Parse(t.Result.GetResponseText());
                        Toast.MakeText(this, string.Format("Welcome {0}.", obj["name"]), ToastLength.Long).Show();
                        StartActivity(typeof(RememberListActivity));
                    }


                }, UIScheduler);
            };

            var intent = auth.GetUI(this);
            StartActivity(intent);
        }