Web audience measurement firm comScore Networks said No. 3-ranked Microsoft's share slid 0.5 percent to 10.5 percent of U.S. Web searches while InterActiveCorp's Ask.com's share dipped 0.1 percent to 5.4 percent.
Google has gained share in 16 of the last 17 months in the United States, the world's largest Internet market, according to comScore data.
Yahoo's share of the market rose 0.3 percent to 28.5 percent in December over November, the survey found.
An estimated 6.7 billion searches were conducted by U.S. Web users in December, up 1 percent from November.
The number of U.S. Web search queries has grown 30 percent since December of 2005, comScore said. Consumers performed 3.2 billion searches on Google sites, while Yahoo drew 1.9 billion searches.