After Microsoft announced Bing a few months ago, it began increasing market share. However, its string of increases halted in September. Microsoft now has its work cut out for it to get more share.
One of Microsoft's biggest challenges will be what turns out to be one of Google's smartest moves - the Google search toolbar. More and more people today don't bother going to Google's home page to begin their searches. Rather, they just type their search request into the Google search toolbar from whatever page they are on. Virtually nobody who searches this way is interested in going back to the old procedure of jumping to an explicit search page - it's wasted navigation. So, the share of the market who uses the toolbar is not even in play for Microsoft to win over. Unless Microsoft comes up with a strategy to address these people, they will be fighting an increasingly difficult war.