Despite fears of an economic downturn, online advertising is continuing to grow. But that growth hasn't been driven by leading brand advertisers, according to Randall Rothenberg, president of the IAB, who told The Washington Post that the web has been "growing by grabbing the low-hanging fruit."

Rothenberg's statement comes just one day after the IAB released its latest quarterly report. The numbers were good -- it was the second highest quarter ever with an 18 percent year-over-year spike -- but the source of those ad dollars isn't as encouraging as many had hoped.

So what kinds of companies are spending a lot of money online? According to The Washington Post article, it's not household names like Procter & Gamble, which devoted just two percent of its $5 billion ad budget to digital in 2007. Instead, big players in online advertising include brands like the University of Phoenix, which reportedly lays out $20 million a month for online advertising.

So far, the bulk of the big brand dollars that have come into digital have been spent on search, leaving display and other opportunities to companies like the University of Phoenix. But as the web continues to mature, many insiders are comparing today's internet to television in the 1950's, which means that big brands may soon start spending more on high-quality content. But so far, those campaigns have been largely experimental.

Most recently, Best Buy, Office Depot and the U.S. Navy all initiated mobile web campaigns on a limited basis, and the tone from brand marketers has been a steady stream of similar tests. Meanwhile, publishers and agencies have been working feverishly to create advertising environments that will entice major brands. But to do that, many expect that the industry will need to move beyond the banner ad. That could mean big opportunities for advocates of emerging platforms like video and social networking. But before that happens, the industry will need to figure out how to monetize those rapidly expanding sectors.