In May, Yahoo was on the top with a 15.9 percent share, compared with 13.5 percent for FIM. The latest data raised concerns about Yahoo!'s ability to maintain its online advertising business.

“It's a continuation of this trend that eyeballs are going to other places on the Internet, and advertisers are recognizing that. It's a step in the wrong direction for where Yahoo wants things to go,” said Piper Jaffray analyst Gene Munster.

The figures also show the potential of MySpace, the social networking site accounting for almost all of FIM's display ad revenues. Social networking sites, such as MySpace and Facebook, have struggled to monetise at the rates they originally expected. Even Google, which places search ads on MySpace, once said that “making money off social networks is more challenging than anticipated,” Smart Money reported.

According to Munster, Yahoo's ad rates were still about five times greater than those of social networks because their users are typically engaged in socialising with friends rather than searching for information or products.

News Corp. said earlier this month that FIM's fiscal 4Q revenues were up 23 percent from a year earlier to $225 million, due to strong search and ad revenue gains. It also noted that interest in branded display ads was strong in the current quarter, while half of all orders included “hyper-targeting,” a new MySpace technology to deliver ads to a specific target group, according to Smart Money.