Increasingly, marketers are looking to ad networks that sell display advertising across targeted groups of websites. Some networks have on offer targeted advertising. Others, termed vertical ad networks, comprise websites which focus on one subject, such as travel, sports or entertainment. Their growth could well mean a lower share of advertising for leading content portals, particularly those considered strong in traditional display advertising.

In 2007, the US revenue growth slowed a bit at three of the four leading portals, according to a detailed analysis done by eMarketer. In fact, any downturn could prove to be bad news for media websites, which attract a lot of display advertising at premium rates.

In the US, billions of dollars are spent on online advertising every year, and the figure is growing. Search advertising, which is Google’s stronghold, corners the majority of that ad spending, according to analysts. It is estimated that the leading 20 ad networks earned close to $2 billion in 2007, or nearly 14 percent of the display market.