Another milestone for Facebook on its slow march towards world domination. According to analytics company ComScore, Facebook has overtaken search-centric Yahoo! to become the world’s third largest website (when measured by unique visits). The social networking site is raking in just under 650 million unique visitors each month, trailing only Microsoft and Google. If it is any consolation to Yahoo! they still rank first in unique visitors in the U.S., although… Facebook does account for nearly 25% of all online ad impressions served in the States — over twice as many as Yahoo!.