Until a couple days ago, Google's stock looked recession proof. Despite mounting talk of an advertising slowdown, shares hit an all-time high in November. Now, it finally seems that recession concerns are catching up with the web giant. Although Google and Yahoo are in different competitive positions (Google could eat Yahoo for a mid-afternoon snack), they are both dependent on advertising for the majority of their income, and as a result, both are poised to take a thunderous hit in the event of a recession.

The key difference between Yahoo and Google is that nobody has high expectations of Yahoo. Its problems have been widely chronicled: Former CEO Terry Semel was sacked last spring; its search business is troubled; and now Yahoo is reportedly planning layoffs to keep its finances in check.

On the other hand, investors and customers alike expect great things of Google. The tight-lipped, overachieving company is notoriously uncommunicative with the financial community, but its silence is usually interpreted to mean that business is growing at mind-blowing rates. Over the last two years, Google's earnings handily exceeded analysts' expectations in six of the eight quarters. Now, with a looming recession and nervousness surrounding the online advertising market, at least a couple analysts think that Yahoo's woes are not unique to the company and could drag on Google, too.

"Advertising is linked to the economic cycle, and the market has come to the conclusion that Google will not be immune to a recession," says Laura Martin, an analyst with Soleil - Media Metrics. "I think what's going to happen is [on the earnings conference call,] somebody will ask about the outlook, and I would expect [Google management] to be conservative. And the market is not going to like that."

The market may already be anticipating the bad news: Over the last two days, Google shares have dropped more than $58, closing at $548.62 on Tuesday. How long before the stock drops below $500? At this rate, it'll be a day or two.