Looking at the statistic in blockchain, there are 146M addresses with >0 BTC. This will be the upper limit.
As we know, almost all bitcoin users have more than 1 address with >0 BTC; I myself have at least 50 addresses with balance >0 BTC. Some enthusiasts and organizations have thousands of these addresses. My guess is that we are still <1 million users, but not far from that.
This is the most logical way to get a range of how many users there really are.