I'm not sure. You don't buy facebook apps, you just use them if someone else invite you to. Is it possible that the advertising revenue is deep enough to support a 50+ employee company?

Yes, in fact web advertising has proved to be deep enough to support all of Google!

Of course there is a limit to the total amount of advertising dollars available and many half-good companies may not be good enough to support themselves with advertising, but there is no question that a good company can do very well on advertising alone.