Even Google Is Vulnerable

Google's growing exposure to international revenue could also help it during a recession in the U.S. During the third quarter, 48% of the company's revenue came from outside the U.S., compared to 44% for the same period a year ago.

Still, a downturn could hamper Google in its push into new markets. The company needs new markets to live up to Wall Street's lofty growth expectations. Even after a week-long drop of 14% to $641.67, shares currently trade at a price-over-earnings-to-growth ratio of 1.23, which means the company is expected to grow annual earnings by almost 35% for each of the next five years.

That sort of growth may seem easy for Google, given that net revenue grew 62% in its latest third quarter. However, that's down from 2006 when the company was posting year-over-year growth of more than 70%.

That means it will have to look well beyond the search ad market.

Two of the key markets that Google is hoping to storm into next -- the display ad market and the video market -- would be likely to feel the effects of a downturn more strongly than search. Google hopes to enter the display business through its proposed acquisition of DoubleClick and introduced video ads for its YouTube video-sharing service in August.

While more measurable than traditional media like television or newspapers, both forms nonetheless focus on the kind of brand-building that advertisers are quicker to pull back on in a downturn.