Microsoft's Bing search service overtook Yahoo! for the first time in August in the United States. Bing accounted for 13.9 percent of all US-based web searches in August, up .25 percent from July. But that was enough to oust Yahoo! from the number-two spot; Yahoo! accounted for just 13.1 percent of US-based searches that month, a stunning drop from its 14.6 percent share in July.

The flipping of search share in August follows a year-long reversal of fortunes for the two companies. Looked at year-over-year, Microsoft's search properties increased US share from 10.7 in August 2009 to 13.9 percent this year. Meanwhile, Yahoo! has fallen from 16 percent to 13.1 percent. (These figures are from market researchers at Nielsen.)

Coincidentally, Yahoo! completed its transition to Microsoft's Bing back-end in August, as well. Yahoo!'s US- and Canada-based sites are now using Bing under the covers, though the look and feel hasn't changed for users. Combined, Bing and Yahoo! now account for about 27 percent of all searches in the United States.

Of course, Google continued its domination of the search market, even when compared with a combined Microsoft/Yahoo! effort. Google controlled 65.1 percent of the US-based search market in August, according to Nielsen, over double the combined usage share of Microsoft and Yahoo!

Although it has made small gains in desktop search, Microsoft isn't yet seeing similar results in the increasingly important mobile front. Its share of US-based mobile search users—those cell phone owners who utilize search services over the air—grew from 1.2 percent in August 2009 to 1.7 percent this year. But Google jumped an even bigger amount, from 9.3 percent to 14.4 percent. (The majority of cell phone users have presumably never used search at all.)

Microsoft signed a search deal that will put Bing on many Verizon mobile devices over the next few years, including, curiously, some Google Android-based phones. It has also created a mobile app version of its Bing service for both Apple iPhone and Android devices.