A lot of them are business accounts, bots and fake or throwaway. I have 3. How can anyone be sure that each account is a person. It says a lot about general IQ on HN that I got 4 downvotes for suggesting that Facebook has less users (real ones) than China has people (real ones, too!)
So you're telling me that if I state a fact but don't back it up then I get down-voted? On so many occasions I've stated facts without explanation that got up-voted. So what I'm trying to say is: the idiocy of crowds.