Market researchers at comScore this week said that Microsoft's Bing has surpassed Yahoo! to become the number-two most popular search service in the United States. And though both firms significantly trail market leader Google, the combined services—Bing powers Yahoo! search as well—now represent a viable alternative to the Google near-monopoly.

"Google Sites led the explicit core search market in December," the comScore report notes, "followed by Microsoft and Yahoo!" AOL was a distant fourth.

comScore says that Bing represents about 15.1 percent of all searches in the United States, a small jump from the 15 percent it commanded about a month before. But that was enough to place it ahead of Yahoo!, which fell from 15.1 percent in November to 15 percent in December. Google continued its dominance, jumping from 65.4 percent of the market to 65.9 percent in the most recent report.

Of course, with Bing powering Yahoo!'s searches, the combined Bing and Yahoo! numbers are perhaps even more relevant. The two firms together account for over 30 percent of all US-based searches, almost half the number Google provides. comScore says that more than 18.2 billion "explicit core searches" were conducted in December, a jump of 2 percent. Meanwhile, "total core search queries" jumped .4 percent to 20.5 billion.