Nielsen Online, an Internet marketing research company publish at 9th March 2009 a report which shows that "social networking" sites are the four the most popular Internet activity, more popular than a personal e-mail.

From their research for markets in Australia, Brazil, France, Germany, Italy, Spain, Swiss, UK and USA you can read that personal e-mail use 65,1% of people that surf the Web, and 66,8% use social networking sites. The most popular site is Facebook.

In their report their also point the fact, that more relevant for growth social networking sites have mobile phone users, which increase their activities in Internet. In 2008 a number of people that use social networking sites from their mobiles growth 249% in UK and 156% in USA.

Full report you can download here:
http://www.nielsen-online.com/pr/pr_090309.pdf