The catch, according to some, is that much of the money flowing toward the Internet is concentrated on a few dozen of the most popular sites. That has left smaller, less well-known sites at a severe disadvantage when it comes to attracting advertising money and surviving.

In the United States, the top 50 Web sites accounted for more than 90 percent of the revenue from online ads in the first half of 2007, according to the Interactive Advertising Bureau and PricewaterhouseCoopers. The top 10 sites accounted for 70 percent of the revenue.