After Yahoo and Microsoft finally integrated their massive search technology and advertising partnership, ComScore is reporting that Yahoo's explicit core search share in the U.S. had declined in December.Yahoo's 16 percent share was down from 16.4 percent in November, which was actually down from 16.5 percent from October. Meanwhile, Microsoft's aggressive Bing search service rose from 11.5 percent in October to 11.8 percent in November to 12 percent in December. And big guns Google more than regained a small loss from October (66.3 percent) to November (66.2 percent) in December, with a 66.6 percent share. (Yes, all you conspiracy theorists: 6, 6, 6.) Yahoo's decline in explicit core search--which ComScore noted "excludes contextually driven searches that do not reflect specific user intent to interact with the search results"--is, of course, a key metric to watch at the company, which needs to improve its revenue growth.