I guess, it would be hard to figure it out on the numbers of actual bitcoin users since there are alt users, making new accounts,.
Figuring it out on how many are totally existing account with per users will be more precise on how many people are using bitcoins.
At this point it will be impossible but that statistical count will be possible in the future. When government will regulate the use of bitcoin and there will be only one registered wallet per individual then an actual count is easy. Aside from gathering statistical datas, scammer and hackers will be easily identified and it will provide more security to users. But I hope if the government places regulation they will not place huge taxes on bitcoins.