Since Facebook has the ability to asynchronously measure the amount of time the page is open (rather than just the amount of time you're actively requesting new

Since Facebook has the ability to asynchronously measure the amount of time the page is open (rather than just the amount of time you're actively requesting new pages) I expect this number somewhat overestimates the amount of time that people are actually focusing on Facebook. I leave email clients, twitter, etc open a lot. By the raw numbers, it could be said that I spend more than half of my waking life using an email client.