Debate may continue about the pros and cons of ad networks, but amid the shaky economy, one thing is certain: Ad networks are in trouble.

As online spending slows down, many networks are feeling the pinch. JellyCloud and Adzilla recently shut down, and AdBrite was forced to cut 40 percent of its workforce, according to The Wall Street Journal. Other networks "are in severe trouble and could be closing their doors in the back half of this year or the beginning of '09. People are bracing for the worst," said Ross Sandler, an internet analyst at RBC Capital Markets.

With online ad budgets tightening, networks have seen potential deals die on the vine, with advertisers even cancelling deals to get a better rate elsewhere. And while JellyCloud, Adzilla and Adbrite may not be household names, the larger networks are feeling the woes as well.

AOL's Platform-A, the largest U.S. ad network, has seen drops in autos, financial and travel ads. Meanwhile, analysts predict ValueClick, the fifth-largest ad network, will see a 2 percent decline in unique visitors this year, and visitors could drop off 7 percent more by the end of 2009.

In an industry clogged with more than 300 competitors, some networks, like Burst Media and Collective Media, are putting themselves up for sale, in the hope that joining a larger network will ensure their survival.