Can you guess the 10 most popular websites in the United States (as of January 24, 2011)? To get that data, we turned to Alexa.com, which ranks sites based on traffic. While their rankings aren't perfect, the "Alexa number" is a common metric when comparing things online. Mentalfloss.com is #4,905 (#1,561 in the U.S.), but we won't make you count that high. You can just enter the site's name (Google), not the URL (www.google.com).