New numbers from comScore Networks show Google pulling further ahead of Microsoft Bing and Yahoo in the U.S. search market in May, claiming its largest lead over its competitors since December 2010.

Google climbed to 66.7 percent of the market during the month, according to the comScore numbers. That was an increase of just 0.2 percentage points from April, but all told, Google has risen by 1.3 points over the last six months. Microsoft remained steady at 15.4 percent of the market, but Yahoo continued to fall, hitting 13.4 percent, down from more than 16 percent at the beginning of 2011.

Microsoft handles the underlying search technology for Yahoo under a partnership between the companies. Combined, their market share slipped to 28.8 percent for the month, almost 38 percentage points behind Google, and down from their market share of more than 30 percent six months ago.

The search companies have been changing their results pages in a bid to attract more users, with Google adding a “Knowledge Graph” feature that summarizes key content in the sidebar, and Microsoft incorporating data from Facebook and other social networks into a sidebar related to the queries people make.

Yahoo released its new Axis “search browser” on May 23, but the new app and browser plugin weren’t enough to reverse the company’s slide.

PreviouslyFacebook, Twitter and friends reshaping search, but Google still rules the roost

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