"Bing hit a milestone this month, reaching 18 percent of search engine market share. It still continues to gain share slowly over time, but for the most part it seems to be gaining from Yahoo, it's partner that runs Bing under the hood..."
Google Sites 66.9% (Unch)
Bing and Bing Powered 29.3 (Unch)
-Microsoft/Bing (Core) 18.0 (+0.1)
-Yahoo! (Bing Powered) 11.3 (-0.1)
Ask Network 2.5 (-0.1)
AOL, Inc. 1.3 (Unch)
Yahoo made a deal with Microsoft several years ago to use the Bing back-end to power its search results. Thus, when a user searches using Yahoo! Search, he is actually getting the results he'd get from searching at bing.com. It seems users have decided they'd rather get the real thing, as Bing has slowly grown since it was launched in 2009 (From roughly 7% then to 18% now). Most of that growth has come from Yahoo.