From the New York Times:

The average time that users spend on Facebook is nearing an hour. That's more than any other leisure activity surveyed by the Bureau of Labor Statistics, with the exception of watching television programs and movies (an average per day of 2.8 hours). It's more time than people spend reading (19 minutes); participating in sports or exercise (17 minutes); or social events (four minutes). It's almost as much time as people spend eating and drinking (1.07 hours).

And, sadly, all of this Facebook time isn't making us happy. A study summarized in the American Journal of Epidemiology concluded: "the more Facebook hours logged on the social network over time, the more their sense of well-being and happiness declined."