Social networks and blogs account for nearly a quarter of the time Americans spend online

"Highlights of Nielsen’s “State of the Media: The Social Media Report”
Social networks and blogs continue to dominate Americans’ time online, now accounting for nearly a quarter of total time spent on the Internet
At over 53 billion total minutes during May 2011, Americans spend more time on Facebook than they do on any other website
Tumblr is an emerging player in social media, nearly tripling its audience from a year ago
Nearly 40 percent of social media users access social media content from their mobile phone
Internet users over the age of 55 are driving the growth of social networking through the Mobile Internet
70 percent of active online adult social networkers shop online, 12 percent more likely than the average adult Internet user
Across a sample of 10 global markets, social networks and blogs are the top online destination in each country, accounting for the majority of time spent online and reaching at least 60 percent of active Internet users"
Source:  Data from Nielsen's 'State of the Media:  The Social Media Report', quoted on NielsenWire, 11th Septeber 2011
See the full report here