Just happened across Facebook's statistics page.
A couple of thoughts:
They have over 500 million active users, and 70% of users are outside the US. If the latter stat refers to active users, that means that the number of Americans on Facebook is very roughly 30% of 500 million, or about 150 million. The total population of the US was 307 million people in mid-2009 (and that presumably includes children too young to use a computer).
Which means that roughly half of the entire US population is on Facebook. If I'm reading their stats page right, and if I'm not making mistakes in my calculations.
(Presumably it's a significantly higher percentage of the US computer-using population; as of mid-2008, 20% of US households didn't have Internet access, though that number has likely dropped a fair bit in the past two years (it was 29% in 2006). Then again, that survey was focused on email (which I gather is becoming increasingly irrelevant to today's youth); and one-third of the US population uses the Internet at public libraries, and the most popular use is social networking. So I'm not sure how big the US non-computer-using population is at this point.)
Second: “People spend over 700 billion minutes per month on Facebook.” That's about 1400 minutes per month per active user, or 23 hours a month, or about 45 minutes a day.
And going in the other direction: There are about 43,800 minutes in the average month, and there are about 6.7 billion people in the world. So humanity's total amount of time to spend per month is about 293.7 trillion minutes.
Which means that roughly 0.2% of all of humanity's time is spent on Facebook; call it three and a half minutes per day per living human. That doesn't seem like much when you put it that way, but it's not often that I come across specific activities that can usefully be thought of in terms of percentage of all of humanity's time spent on them. (Beyond the basic activities that've been around for centuries or millennia, I mean.)