The Motley Fool has a good article about why Google isn't worth as much as people think.
Today, Wall Street's expectations for Google are sky-high. Even a small misstep has the potential to cause current stockholders considerable pain. Analysts currently estimate 30.5% annual revenue growth for the next five years, placing 2011 revenues at about $24.4 billion. That's larger than WPP Group, and Omnicom, two of the largest advertising agencies in the world today, combined. It's a precarious perch for a company facing a number of serious challenges.
Personally, I find myself using Google less and less as time goes by. Search is a sexy business that the top tech companies all want to own. That kind of competition can be bad news for profits.