The lead is tiny - 466,000 visitors out of about 141 million apiece. In fact, based on rankings from Nielsen Online, Google has already been on the top.

However, comScore's finding is another evidence of Google's dominance over Yahoo, the Associate Press reported.

Jack Flanagan, comScore's executive vice president, pointed out that Google's dominance in search creates "a halo effect" that can boost its other services.

Google now has the online photo-sharing service, Picasa, competing against Flickr from Yahoo. Google also has launched a finance site Yahoo to vie against Yahoo.

Regarding video-sharing, Google's YouTube is already on the lead.

According to comScore’s data, Google's U.S. unique audience in April reached 141.1 million, an 18 percent increase from the same month last year. Yahoo's audience was 140.6 million, up seven percent, while Microsoft was third at about 121 million.

However, Yahoo still leads in page views, which means visitors spend more time there or return more often. According to the article posted on Yahoo News, many Google users simply make a search request and leave quickly to the result sites. Yahoo gained 33.6 billion page views while Google had 28.7 billion.