The actual number of people using bitcoin is not possible to determine.
Even if you track the number of accounts, then also it is not possible because, one user can generate lot of accounts.
Only if you can track the IP of the users then you might get an estimate. But there's no easy way to do it.
that is true. a single user uses multiple accounts and it becomes difficult to know the number of persons using bitcoin. but it is sure that bitcoin is spreading among everyone.