You're dividing minutes per month by the total population of the USA (appx. 300 million) to get "avg. American," but I'm sure Facebook has more unique users tha

You're dividing minutes per month by the total population of the USA (appx. 300 million) to get "avg. American," but I'm sure Facebook has more unique users than Tumblr. So the disparity in time spent on site per person probably isn't so much.