From the Nielsen Company:
Facebook and Twitter have post large year over year gains in unique users.
Compared to user activity on social networking sites last year, Facebook and Twitter posted gains of 69% and 45% respectively in the U.S.
Globally, the total minutes spent on social networks monthly saw a more than 100% gain over the same time last year, driving the average time per person spent on social networks to over six hours per month.
I think it would be interesting to see a distribution graph illustrating the difference between how much time the top users of social networks spend each month compared to those who use spend the least amount of time.