Google leads the way in an October 2011 study by comScore:
Google led the U.S. core search market in April with 65.4 percent of the searches conducted, followed by Yahoo! with 17.2 percent, and Microsoft with 13.4 percent. (Microsoft powers Yahoo Search. In the real world, most webmasters see a much higher percentage of their traffic from Google than these numbers suggest.)
Americans alone conducted a staggering 20.3 billion searches in one month. Google accounted for 13.4 billion searches, followed by Yahoo! (3.3 billion), Microsoft (2.7 billion), Ask Network (518 million), and AOL LLC (277 million).
Total search powered by Google properties equaled 67.7 percent of all search queries, followed by Bing which powered 26.7 percent of all search.