I have a Facebook Web Game that I converted to Android and iOS using PhoneGap. I can use Facebook for authentication on my mobile apps, but I would also like to allow the user to play as a guest. I've seen various games implement this workflow before and I'm curious how it is actually done. What unique identifier is the phone sending to the server to authenticate against? Does each phone have something unique about it that PhoneGap can expose via a plugin? Does anyone know the logic behind the "Play As Guest" feature many mobile games implement?