For the first time, MSN/Windows Live/Bing Search overtook Yahoo! as the #2 search engine in the U.S. with a 13.9% share of search volume in August 2010, a 0.25% delta increase from the previous month. Although Google saw little change in its month-over-month search volume, it still dominates the search market, accounting for 65% of all U.S. searches, according to Nielsen research.

Yahoo! followed Google and MSN/Windows Live/Bing Search with a 13.1% share of U.S. searches, falling from a 14.3% share in July 2010 (a 1.2% delta decrease or an 8% relative decrease).

In terms of a year-over-year comparison, Google has seen little change in its share of search while Yahoo! has seen a small but steady decline, going from a 16.0% share to 13.1% (a delta drop of 2.9% or a relative drop of 18%). MSN/Windows Live/Bing’s share has grown from 10.7% in August 2009 to 13.9% (a delta increase of 3.2% or a relative increase of 30%).

Microsoft and Yahoo announced a search deal in July 2009 where Microsoft would start powering Yahoo! Search while Yahoo! became the exclusive worldwide relationship sales force for both companies’ premium search advertisers. Microsoft Bing officially started powering part of Yahoo! searches starting in August 24, 2010. If we combined Bing-powered search in August pro-forma, it would represent a 26% share of search.