There are other people that does not belong to this forum that involve in bitcoin like the investors who invest on it. Many people who use bitcoin want to hide their identity because it is also used in illegal activities! from that alone it's very impossible to know how many numbers of bitcoin users. These people is that they also enjoy buyers protection. I don't think we'll ever come to a point where even a small or medium country is feeling the impact of people transacting with Bitcoin. I think by collecting every wallet and registered account can predict the number of each user, even if it does not really match. because everyone can generate some wallets and accounts.