Source: http://blog.compete.com/2011/06/16/may-2011-search-market-share-report/

• Search query volume across the 5 engines picked up slightly in May (up 2.5% from April), driving small shifts in search market share.
• Google’s share of the search market declined by 0.2ppts, although its query volume increased by 2.3%.
• While Microsoft’s share of the search market declined by 0.1ppt, the growth of Yahoo! by 0.5ppts resulted in an increase for Bing Powered engines M-O-M.
• Yahoo! experienced the largest growth in queries, driving a 0.5ppt M-O-M increase in share.
• All 5 engines saw slight increases in the number of unique visitors from April to May except for AOL which remained flat.