We just registered our two millionth person. I've just noticed something interesting--many sites have lots of users with multiple accounts. I would not venture a guess, but from experience on our forum, which is not that active and has been up for a couple months and is not included in our numbers - I would say probably 50% are people with more than one account - going to a service like myspace, it's not hard to imagine that half the users are spam accounts. Probably the same at content sites like digg. Some sites let users create mutliple accounts with one email address, or even - no email address. Our policy is a simple 1 user = 1 email address. We really try to keep things simple here. Fighting off the bots is a little challenging.