Some interesting news about Social Media Sites in a nutshell:

Nielsen Wire (May 2009):

• People continue to spend more time on social networking and blog sites than ever before, with total minutes increasing 82% year-over-year and the average time per person increasing 67% year-over-year in May 2009
• Twitter.com was the fastest-growing Web brand in May 2009, increasing 1.448% year-over-year, from 1.2 million unique visitors in May 2008 to 18.2 million in 2009

• Facebook overtakes MySpace globally and in the US:
• Globally: With 144.3 million unique visitors, Facebook was the No. 1 global social networking destination in May 2009—the 7th month in a row that Facebook has ranked No. 1 after dethroning MySpace
• USA: May 2009 also marked the 5th month in a row that Facebook has been the No. 1 social networking site in the U.S., garnering 75.4 million unique visitors—a 190% increase over May 2008

eMarketer shows us the corresponding charts [with slightly different numbers]:

• According to comScore, Facebook totaled 70.3 million unique visitors, up 97% from May 2008 to May 2009. MySpace hits shrank 5% over the same timeframe, fading to 70.3 million unique visitors
• 147 million of the total 194 million internet user are registered in Social Networking Sites