Four entrepreneurs started a company in 1998 to help Internet publishers manage advertisements on their sites. They took the company public and then sold it to DoubleClick in 2001.

The team of four, friends by then, went to new and different challenges at online ventures.

But recently they figured the same market conditions that existed back in the late ’90s still exist. Now they’ve come back together to launch Rubicon Project, a West Los Angeles company that manages ad network sales across small Web sites.

After they sold their earlier company, called L90/adMonitor, Frank Addante started a wireless company in 2001 that failed. Then he self-funded a company called Strongmail Systems, and he remains that company’s largest individual shareholder.

Craig Roah built and sold another company, then dabbled in real estate. Duc Chau, the programmer behind adMonitor, went on to write the original code for MySpace which wasn’t used and he then helped develop another social networking site. And Julie Mattern developed programs for online ad companies including FastClick, ValueClick and Euroclick.

With their latest creation they hope to bring order to the increasingly fragmented and inefficient market for online advertising.

Addante said that while Internet business has grown more Web sites, more ad impressions, more traffic, more advertising dollars its structure hasn’t really matured. In particular, the four friends noted the growth of ad networks, which are groups of numerous small Web sites that offer their combined traffic to advertisers.

“Seven years ago, when we were in this business, there were 15 ad networks. Two years ago, there were over 100 today, more than 300,” said Addante. “If you’re a Web site and you want to make the most money from your ad space, you have to connect to all the different funding sources. If advertisers are spending money in 300 different places, Web publishers need to access all of them. It gets very complex.”

After the AdMonitor acquisition, they worked as consultants on each other’s ventures and served on each other’s boards. But they didn’t get together regularly.

The group came back together when Addante saw that Internet advertising wasn’t evolving as quickly as it was growing. He and Roah had a few lunch discussions about stagnation in the industry.

“He started talking to Julie and Duc and the next thing you know we had a couple of Starbucks breaks,” Roah recalled. “We all went back and did our own research. That’s when we realized that the same problems that existed seven years ago were still there, and no one was solving these problems.”

Chau was going to move back to China, but changed his plans when the group decided to greenlight the company. “Ultimately, the opportunity to do what hasn’t been done yet drew me back to stay here,” he said.

At Rubicon, the entrepreneurs have assumed their familiar roles. Addante serves as chief executive officer, Roah as chief operating officer, Chau as vice-president of engineering and Mattern as vice-president of technology.

“This company underscores that it’s all about the team,” Addante said. “I don’t mean a collection of smart individuals, but a group of smart individuals who work well together. A great team attracts other great people, and we’re seeing that now.”

And it seems they still have the old magic. During its first day in beta, 500 Web publishers signed up for the Rubicon Project. So far, more than 1,000 sites have used the technology to process half a billion ads.

Meanwhile, the number of networks continues to proliferate. They range in size from Google’s AdSense to tiny startups. Some concentrate on vertical consumer segments such as women, sports enthusiasts or travel. Others are built around geography and others specialize in video, text or direct response ads.

Tim Hanlon, executive vice-president of the consulting firm Denuo, said the goal of the first generation of networks was to place ads on many small sites. “To appeal to the big brand advertisers, you need that scale,” he explained. Now, the concept is maturing.

“The irony is there’s no shortage of ad networks being created,” Hanlon said.

So Rubicon, which has 27 employees, hopes its technology can bring order to this expanding world.

When a Web site has an unsold ad space available, it sends a message to the Rubicon server. The company’s software selects the best ad network for this particular opportunity and instantly connects it with the site, resulting in an ad appearing on the user’s screen.

The four entrepreneurs still don’t have an end-game strategy for their latest venture. “We see the opportunity to be No. 1 in this space,” said Addante. “We’re not thinking about exits. We’re focused on solving these big, massive inefficiencies. We just want to fix the online advertising space.”