According to the latest results of Search Engine market share rankings in the U.S. for December 2011 compared to November 2011, it appears that Microsoft’s Bing have finally ousted Yahoo! for that coveted second spot. The data collected by comScore shows that, Yahoo managed to lose 0.6 percent of their market share bringing their total down to 14.5 percent, and it turns out that 0.1 percent of that went to Bing whose current share sits at 15.1 percent.
And as you would expect, Google increased their domination with Search over the holiday period by gaining a 0.5 percent from Yahoo, effectively taking their total to 65.9 percent. There also seems to be no change in share from the Ask network (2.9) and AOL (1.6).

Source: comScore