Bing grabbed 12.9 percent of the U.S. market versus Yahoo's 10.15 percent share, but both are still well behind market leader Google at 74.99 percent, reported StatCounter, which bases its research on an analysis of 1.316 billion search-engine referring clicks -- including 336 million from the U.S.

"While (Bing's) lead over Yahoo may not last into next week, our data suggests that it is slowly but surely closing the gap," said StatCounter CEO Aodhan Cullen.

At Google's Expense

StatCounter reported earlier this month that Bing helped Microsoft increase its share of the U.S. search market by one percentage point during June. Overall, Microsoft held an 8.23 percent share last month, trailing far behind Google (78.48 percent) and Yahoo (11.04 percent).

Cullen said the latest data indicate Bing's success is coming at Google's expense. "We can see that Bing is gaining very slowly, but it is gaining, and the data is almost a mirror image in that when Bing goes up, Google goes down and vice versa," Cullen said. "But Yahoo remains very steady -- it's not losing any share."

Other recent reports suggest that Google users in particular seem to be looking for a viable alternative to the current search-engine market leader.

"In the first week following the announcement of brand Bing -- but still before the official June 1 launch -- 97 percent of visitors to Bing.com overlapped with Google, compared with only 37 percent and two percent overlap with Yahoo and AOL, respectively," said Taylor Holsinger at Web analytics firm Compete.com. "During launch week, the dramatically larger overlap of Bing Googlers continued, relative to the Bing overlap with Yahoo and AOL."

Bing Tweaks

However, Cullen cautioned that Bing's rise in popularity also may be getting a temporary boost from recent press reports touting the search engine's advantages over Google. "The jump in Bing's share may reflect a positive review of the search engine compared to Google which appeared online in The New York Times on the 8th and in the print version on the 9th July," Cullen said.

Among other things, the Times article pointed out that the first few paragraphs of a story can be accessed at Bing by hovering the mouse arrow over a link. By contrast, Google users have to click on the link and access the story directly to learn more.

As for what happens to Microsoft's new search engine in the days ahead, "we just have to wait and see if Bing will fall back down again, and it probably will," Cullen said. "But overall Bing is gaining traction."

Meanwhile, Microsoft is expected to continue tweaking Bing in ways that further differentiate the new search engine from Google. For example, earlier this month the software giant added Tweets from prominent Twitter members to the content displayed by its search engine.

In its initial foray into integrating more real-time data into Bing's search results, Microsoft is "starting with some of the more prominent and prolific Twitterers from a variety of spheres," said Sean Suchter, general manager of Microsoft's Silicon Valley search technology center. "We picked a few thousand people to start, based primarily on their follower count and volume of tweets."