Google was able to extend its search market share by more than 10% in 2007 and was estimated by Nielsen Netratings at 56.4% of the total U.S. search market in December. Google was able to increase its share from January to December by 5.5 points, while the company’s main rival, Yahoo dropped 5.4 points from 23.1% to 17.7%, according to the market research firm. Microsoft came in as a solid third at 13.8% share, up from 8.4% in January 2007.

Interestingly, ComScore disagrees on Yahoo’s market share and ranks the company higher: The firm estimates Yahoo’s share at 22.9% (down from 23.5% in July 2007 when ComScore began publishing search market shares) and Microsoft at 9.8% (down from 12.3%). Google also dominates ComScore’s chart with a 58.4% share (up from 55.2% in July).