Twitter is huge. Facebook is enormous. MySpace is still a serious powerhouse. The bottom line? Social networks are a force to be reckoned with, and Americans are spending lots and lots of time on them. In a new study released this week, Nielsen reveled that United States citizens are spending more time than over on these sites. In fact, it found that the number of minutes that Americans spent on these sites doubled (doubled!) in the past year. Crazier still, Nielsen Online discovered that “the number of minutes