Even Google Is Vulnerable

And unlike flashier ads that may help make new customers aware of their products or make a brand more appealing, search ads tend to be more closely linked to customers who are already likely to make a purchase -- and therefore drive sales.

"Our discussions with advertisers indicate that when marketing budgets get trimmed during periods of economic weakness, spending on media that drive sales gets cut last or not at all," Cowen analyst Jim Friedland wrote in a September research report.

Indeed, there is even reason to believe that Google could stand to benefit from a sluggish economy. As advertising budgets get lean, advertisers may rotate funds out of more expensive, difficult-to-measure and brand-building forms of advertising like television into more targeted efforts that can directly boost the bottom line.

Search advertising is a relatively new form of advertising and lacks history to draw upon. But Friedland draws a parallel between direct-mail advertising and search advertising, since both are designed with the intent of driving sales.

And he notes that advertisers have actually increased spending in every recession in the U.S. since 1950, and that the growth rate of direct-mail spending accelerated in five of the last nine recessions.

But Google, which should post close to $12 billion in revenue excluding costs this year, "is growing so quickly that it is unlikely that revenue growth would accelerate in an economic downturn," he says.