I would say 'round 3 million.
How would you know that 3 million people is using bitcoin? Its impossible to know how much of people is using bitcoin cause bitcoin is anonymously used. We cant determined it.Even the wallet refigistrants is an anonymous. Some are making multiple accounts so how would you know? A guess?
Yes, I guess the users are mostly located in Europe, Asia and North America but I don't think they're more than 5 million, so I said 3 millions.