Facebook is the #1 social networking site in the rank. We can't find YOUTUBE.COM or MYSPACE.COM in the rank. Where have they gone?

Source: comScore Media Metrix Ranks Top 50 U.S. Web Properties for June 2009
"It's hard to believe that Twitter.com had fewer than a million visitors in June 2008, and now attracts more than 20 million visitors a month and ranks among the top 50 U.S. properties," says Jack Flanagan, in the report.
Comparing to Facebook, Twitter is rapidly growing. In June 2008, number of unique visitors of Facebook was 37,375,000 and that of now is 77,031,000. Twitter can be the #1 social media in the future if the growth continues at this rate.