According to the latest report from Hitwise Bing powered searches accounted for over 30% of the searches in March. Granted it’s not the 64% that Google produced but it’s still BILLIONS of searches.
Google and Bing use different ranking algorithms and they deliver different websites for the same search query.
Check your analytics. Are Bing and Yahoo! each delivering somewhere near 15% of the visits from search engines? If not you may be having a problem with your Bing optimization.
Another interesting stat published by Hitwise is that is the Bing searches deliver more click-throughs than Google. Bing search resulted in more than 80 percent of searches visiting a website while on Google the visit rate was only 66%. It might be that the search results snippets in Bing are more attractive or the results in Bing are simply more relevant.
You may also want to get a Search Engine Ranking Report.