Yes, but most high growth publicly traded companies trade at 20-40x earnings (profits). Now Facebook has grown tremendously year over year, but this growth has

Yes, but most high growth publicly traded companies trade at 20-40x earnings (profits). Now Facebook has grown tremendously year over year, but this growth has to stop. They already claim 500M active users which is about 1/12th the world's population so their user base is really close to some maximum (3B?). How much higher can their profits actually go in order to match their current valuation?