42.3% of the U.S. Population Uses Facebook

"A new report from eMarketer finds that most adult Americans with Internet access use Facebook at least once a month, and a full 42.3% of the entire American population was using the site as of this month.  

"By contrast, Twitter‘s penetration rate was much lower, sitting at around 7% of the total population and 9% of the Internet-using population, according to the report" (http://mashable.com/2011/02/24/facebook-twitter-number/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+Mashable+%28Mashable%29