According to statistics firm comScore, Microsoft claimed 11.3 percent of U.S. online searches in January, with both Yahoo and Google recording incremental losses that put them at a respective 17 percent and 65.4 percent of the market.

"Overall, the trends continue to be very strong for Bing, and remain very bad for Yahoo," BroadPoint AmTech analyst Ben Schacter wrote in a Feb. 9 research note. "Investors hoping that tonight's data would show stabilization for Yahoo will be disappointed." Since November 2008, Yahoo's market share has declined by more than three points.

Yahoo's continual erosion suggests that the company's multimillion-dollar advertising push, and its renewed emphasis on its end-user functionality, is not performing as CEO Carol Bartz evidently hoped. It also potentially weakens the search-and-advertising deal struck between Yahoo and Microsoft last summer, which will see Bing powering search on Yahoo's sites; if Yahoo users keep jumping ship, then the market share that Bing will receive from its rival/partner will necessarily decrease. Bing's share of the U.S. search engine market could have risen to around 30 percent without attrition; the real number, it seems, will be somewhere south of that.

If Yahoo continues on this downward trajectory, that increases the pressure on Bing to gain market share solely through its own initiatives and features. Microsoft seems dedicated to making its search engine increasingly robust, having closed out 2009 by rolling out several new Bing features, including a beta version of Bing Maps and a more in-depth video-search page.

But the Feb. 9 rollout of Google Buzz, which allows Gmail users to post status updates and swap media such as Picasa photos, suggests that Google is cognizant of the old adage about large companies' worst enemy being complacency; the introduction of that service was not only a direct competitive challenge to the current social-networking primacy of Twitter and Facebook, but also a message to Microsoft that Google will continue to initiate seismic changes to its online lineup in 2010.

I'm expecting that Microsoft and Google will continue to play leapfrog throughout the rest of the year, adding new features and integration with other services. If Bing continues its momentum, it has the opportunity to become the next Yahoo, market-share wise. But Yahoo itself continues to be in very fundamental trouble.