The main reason for the decline, according to the report, is that individuals are searching less: about 7 percent fewer searches per user. That's enough to cause the decline, even though the number of searchers is up 4 percent.
Of the major search engines, only Bing saw an increase in searches per user: 6 percent, ComScore said.
ComScore names two possible reasons for the decline: the move toward vertical search -- Amazon for products, Facebook for people, and so on -- and the shift to mobile devices. Vertical search is ascendant, according to the report, rising 8 percent last year. That should only accelerate in 2013, as Facebook -- where Internet users spend more than 10 percent of all their time online -- continues to develop its Graph Search feature.
"The search market is at a bit of a crossroads with the market having stabilized in many respects, and while it is still extraordinarily profitable, there seems to be a desire to see the market evolve and deliver new value to consumers," the report's authors write.
To the surprise of no one, Google remains the dominant player in U.S. desktop search, and accounts for two out of every three searches. (That's up less than 1 percent over last year.) Bing is No. 2 with 16.3 percent market share, up 1.2 percent. Yahoo is in third place at 12.2 percent, and Ask Network is fourth with 3 percent. AOL has 1.8 percent of the search market.
The good news for Google: It has moved to take advantage of both the move to vertical search and the shift to mobile. Google's share of the mobile search market is estimated at more than 95 percent, and it continually rolls out new features for product search, flight search, and other verticals.