Bloomberg reports that Yahoo has gained search share in September 2008 while Microsoft has lost some of its share.
Yahoo had about 20.2 percent of queries in September, up from 19.6 percent in August, Reston, Virginia-based ComScore said today in an e-mail. Microsoft's share fell to 8.5 percent from 8.9 percent. Google Inc. handled 62.9 percent, compared with 63 percent in August.
Some feel that Yahoo's numbers are grossly miscalculated; one forum member has never seen more than 13% of Yahoo visitors and thinks that comScore's assessment isn't correct. However, pageoneresults notes that Hitwise also does a similar calculation with Yahoo at 18.06% for September. That said, the data is said to be based on "speculative algorithms."
I would argue that different users use different search engines. Maybe they aren't searching for your site/product.