BlogAds founder Henry Copeland said the reason the company began letting bloggers aggregate themselves into smaller networks is because the sheer volume of blogs made it difficult to keep track of them all, and create proposals for advertisers. "We've got over 700 blogs now, and we can't keep track of them anymore--who's who, what they're doing, who's good, who's bad," he said. "When someone wants to spend $20,000, we just get destroyed building a proposal--is this blogger on the left or the right, a movie blog, a music blog, a gay blog, or what?"

With the smaller, blogger-created networks, the bloggers get to decide for themselves, Copeland said, and allowing advertisers to select blogs by categories is much more appealing. "Talking about generic blogs is boring--it's much better if you can go to advertisers and say 'here are the premiere food blogs,'" he said.

So far, 17 networks have been created, including ones for evangelical blogs, law blogs, Philadelphia blogs, baseball, and gay issues blogs.

The theory behind the networks, Copeland said, is that the bloggers can group themselves together much better than a top-down organization could. And when the bloggers organize themselves, he said, advertisers get much easier access to the vaunted "long tail" of the blogosphere--the millions of readers who aren't reading the top bloggers, but are nonetheless very engaged in lower-traffic blogs.

Chris Bowers, who writes for progressive political blog MyDD.com, and helped to organize "Advertise Liberally," BlogAd's liberal blog network, said that the category networks will be very helpful to advertisers looking for topic blogs, but who are unwilling or unable to spend the time to hunt for them on the rapidly expanding blogosphere.

"The blogosphere is enormous, and unless you are extremely familiar with it, sifting through the thousands of different blogs that offer advertising can be extremely difficult," Bowers said. The network model lets advertisers know what a blog is like in general without ever visiting any of the blogs in the network."

Jeff Davidson, who runs Earvolution.com and organized the BlogAds music network, said that the music network helps its sites gain some more clout with advertisers because of their combined traffic. "We wanted to put together a network to leverage our collective readerships to lure in advertisers," he said. "Together we have more monthly page views and readers than many 'famous' music magazines." The music network boasts monthly traffic of over 1.6 million unique visitors, with a month-long ad buy across all 10 blogs costing $2007. RollingStone.com claims 2.5 million unique users per month on their Web site, while Spin Magazine claims 550,000 paid subscribers.