Not only is Facebook now the top source of traffic to Web portal sites, the company has also enjoyed a huge percentage gain in U.S. search market share from Dec 2009 to Jan 2010. While nowhere near the volume of Google searches, it’s clearly a growing activity on Facebook which also reflects Facebook’s new emphasis on search in their new homepage design.

According to comScore, Google still has nearly two-thirds of the U.S. search market, but dropped a fraction of a percent from 65.7% in Dec 2009 to 65.4% in Jan 2010 [source: Information Week]. While Google is still the leader in the search space, and Facebook only accounted for just under 400 million searches in January, that is a gain of 13% over December. If this trend continues, Google may have ample reason to fear Facebook. (Don’t forget Facebook’s supposed plans to launch their own email client, Titan.)

The remaining percentage of the search market for Jan 2010 was divided up amongst Yahoo (17%), Microsoft Bing (10.7%) and various other search engines. Bing gained just over a half percent since Dec 2009, and is up nearly 3% since about a year ago. (Google was at 63%.) Amazon and Fox Interactive Media both lost market share.

Overall, the number of searches from all sources in Jan 2010 is up over Dec 2009, with Google making up nearly 10 billion of the over 15 billion searches. So Google is still leading in U.S. searches, but it remains to be seen whether their newly introduced competitor to Facebook, Google Buzz, will gain enough traction with users to help Google also be the top source of Web traffic to portal sites, or whether they’ll continue to lose ground to Facebook and other established social media sites.