Your logic makes sense, assuming in fact Zynga does have sustainable profits, which might be true (although I guestion whether simple web games are a defensible business, competition easily cuts it).
But, assuming that is true and you are right, does that still justify Facebook's valuation if it is entirely dependent on one company, Zynga? I would conclude that Zynga would be worth quite a lot, but that would make Facebook's position quite a bit weaker, do you agree?
Also, the feedback loop is that Facebook's valuation and success spurs people and investors to start companies and then advertise them on FB. This has been my experience talking to many (not all!) would-be entrepreneurs and investors in new york and the valley. I'm interested in learning about different experiences you may have had.