November 16, 2007 -- The lucrative business of selling Web ads has become so fragmented - and easy to do - that even Martha Stewart has thrown her hat in the ring by setting up an online advertising network.

Driving the growth of these networks is a desire by advertisers to reach niche audiences - from rugby players to homemakers.

Niche "vertical" networks gather together blogs and other Web sites based on specific subjects and then offer to sell ads across them in exchange for a cut of the revenue.

While most of these networks are new players on the advertising scene, big media companies are also getting in on the action.

Stewart started Martha's Circle, a lifestyle network that sells space on a handful of sites including Apartment Therapy, 101 Cookbooks and Style Me Pretty.

Reader's Digest, Forbes and Warner Bros. also launched online networks.

The world of niche networks is populated with companies like Rugby Marketing Global, which represents rugby-related destinations such as American Rugby News and rugbyrugby.com.

Others examples include The Gay Ad Network, which connects marketers with the gay community, and DogTime Media, which puts advertisers in touch with pet lovers.

There's even a company that specializes in getting niche networks off the ground. Adify sells a build-your-own-network platform for companies that don't want the headache of developing their own.

Russell Fradin, Adify's chief executive, estimates there are about 75 niche networks with more popping up each day.

Fradin said networks are proliferating in response to fragmentation on the Web. Six years ago the top five Web sites accounted for 40 percent of the total time spent online, he said. Today, they account for less than 20 percent.

"Advertisers instead of buying [ads on] five sites will have to buy 50," he said.

Although most of these specialty networks are small, a few are getting big enough to rival established players. Glam Media, a growing group of female-focused sites, overtook long-time women's leader iVillage in the Web rankings over the summer.

Creating an alliance of sites can also get the attention of advertisers by boosting a site's combined ranking. Critics argue that networks are simply buying traffic rather than building a Web destination.

This criticism doesn't deter Waterfront Media, a collection of diet and self-help sites such as Southbeachdiet.com and Everydayhealth.com, from gunning for No. 1 WebMD in the rankings.

"That's why you see networks popping up," said Waterfront co-founder and CEO Ben Wolin. "People are trying to deal with the fragmentation and make it so the advertiser doesn't have to maintain 10,000 relationships."