In 2006, ad networks accounted for a meager 5 percent of the ad inventory sold online, but that figure mushroomed to 30 percent in 2007, according to a new study from the IAB and Bain & Company.

The study attributed the surge in ad networks to a lack of adequate pricing tools and inventory management discipline, which in turn has forced many publishers to offload huge amounts of remnant inventory at deep discounts, even at a time when many continue to pump out more content than ever before. However, the study also found that ad networks have benefited greatly from the overall shift to digital as marketers look to achieve greater scale at lower CPMs.

While the overall news points to increasing growth in digital, the study warned that rock-bottom CPMs wouldn't be a good thing in the long-run for either publishers or ad networks.

"Online publishers are producing more inventory than the market demands and risk devaluing the premium nature of their brands, particularly in light of ad networks' growth and their dramatically lower pricing," said John Frelinghuysen, a partner in Bain's Global Media Practice and author of the study. "Building more effective relationships between publishers and ad networks is critical. In the longer-term, both parties will benefit from gains in ad network CPMs."