They already have 400 million users, and their growth might continue. But there are only 7 billion people on earth, they are running out of room to prove that valuation. Even if they literally sign up every single person on earth, that's still more than a few dollars per person in revenue required to make the $20 billion valuation worthwhile.
On the other hand, Google is investing massively in "next big thing" r&d, and even if all its efforts to create new markets fail, it's likely to still be a major player in adtech in 33 years time.
WhatsApp has staggering growth on the basis you don't have to pay for it and its founders are hostile towards advertising, neither of which lend you towards thinking it'll ever reach the 33x P/E ratio (actual P/E ratio >800) still less be significant in 33 years time.
Not sure where to start. What you say isn't even wrong, exactly.
1) You seem to think that the price/earnings ratio has something to do with how the company will do in (P/E) years. It doesn't. As a specific example, companies that are takeover targets have higher PE ratios than they would otherwise, even though they probably won't exist soon.
2) Generally companies with high growth trade at high PE ratios. That's why Google has such a high ratio (it's unclear if you understand this). WhatsApp has even higher growth. AFAIK WhatsApp's revenue is not publicly known, and I'm estimating it at $200m for next year[1].
3) Where are you getting "actual P/E ratio >800"? You know WhatsApp's revenue?
Edit: I see WhatsApp revenue was $20M last year[2]. With growth rates like they are seeing I don't think this says much other than that the revenue model works.
