Even Google Is Vulnerable

And unlike search, where Google is already a market leader, the company is only entering display ads and video now and faces an uphill battle. A slowing economy would only make entry more difficult.

The same goes for Google's ambitions in the mobile arena. The company's announcement of an operating system for mobile phones sparked excitement about the company's stock recently, and Google has frequently talked up the potential of mobile advertising.

But in the event of a downturn, "for Google's truly experimental new-media formats such as search ads on Google mobile search, marketers' willingness to experiment will decrease unless that advertising is sold on a performance basis," says Didit's Lee.

Google's bread-and-butter business will hold up just fine -- and may even thrive -- in a tighter economy. But the same can't be said for the company's stock price.