public static string ClientId = "client id";
public static string ClientSecret = "client secret";
public static string Scope = "email";
public static string AuthorizeUrl = "https://m.facebook.com/dialog/oauth";
public static string RedirectUrl = "https://www.facebook.com/connect/login_success.html";
public static string AccessTokenUrl = "https://m.facebook.com/dialog/oauth/token";

public class AuthenticationPageRenderer : PageRenderer
{
    public override void ViewDidAppear(bool animated)
    {
        base.ViewDidAppear (animated);

        var auth = new OAuth2Authenticator (
            Constants.ClientId,
            Constants.ClientSecret,
            Constants.Scope,
            new Uri (Constants.AuthorizeUrl),
            new Uri (Constants.RedirectUrl),
            new Uri (Constants.AccessTokenUrl)
        );

        auth.Completed += OnAuthenticationCompleted;
        PresentViewController (auth.GetUI (), true, null);
    }

    async void OnAuthenticationCompleted (object sender, AuthenticatorCompletedEventArgs e)
    {
        Debug.WriteLine ("AUTH Completed!");
        if (e.IsAuthenticated) {

        }
    }
}