Advertising. 95% of their revenue is ads. So, the first time we have a downturn, their revenue will drop like a rock. Never the less, they're a cash machine. Their model is extremely profitable, but I think people are definitely overhyping the company. Pretty much all the stuff they do (sans search) is beta, is intended to attempt to keep people coming back, and none of it makes any revenue directly.
Guess what - Microsoft is not going anywhere, and they have far more cash to burn than Google. It'll make for some great tech competition in the next 10 years.
Google is evil - or at least it's founders are.