Well firstly it is ridiculous to assume that their potential user base is nearly fully saturated. There is massive opportunities if they can get China and India onboard. And there is still plenty of growth as technology spreads through the remaining parts of the world.

As for revenue they don't need to have more ads. They just need to continue to provide different ways for advertisers to find the slice of Facebook users they need. Facebook already is much, much better than Google and eventually more and more advertisers will realise this.