According to online researcher comScore’s latest monthly statistics, Google maintained its position as the most important target surface for search engine optimization (SEO) efforts centered on the U.S. market.

However, rival Microsoft continued to slowly inch its way closer to its still-distant rival, posting a 0.6 percent gain in market share to end January 2010 at 11.3 percent. Analysts say that that Microsoft drew approximately half of those new customers from Google and the other half from Yahoo.

Microsoft’s Bing search engine is due to take over search functions for Yahoo’s central online portal, pending regulatory approval of the deal. Combined, Yahoo and Microsoft would account for 28.3 percent of the U.S. search market based on last month’s statistics, edging ever closer to the one-third mark that experts say would make the conglomerate a serious competitor to Google.

A more detailed breakdown of ComScore’s January numbers suggests that most of the decline in Google’s overall site traffic was the result of downturns at YouTube and other Google-owned sites, as the company’s core search engine actually saw more traffic than the previous month.