Therefore, I wonder if the community has come with some approach to analyze the number of actual users that own or make transactions in Bitcoins. This information would be very valuable to explain the validity of the cryptocurrency as a means of payment in the global economy.
A digital currency with user-defined anonymity will cause any estimates to simply be guesstimates. Trends up and down, and to what degree up or down might be easier determined, especially from sampling, but coming up with an estimate of aggregate total number of users (defined as people holding bitcoins ?) is simply not possible. There are other areas where studies have attempted to quantify a population and simply concluded there is no reliable way to come up with the number so rather than provide misinformation there simply is no estimate made.
The upper bounds can be known but the likelihood of the real number of users being anywhere near those bounds is extremely low. For instance, there were several million Instawallets created. However one is created for each HTTP request which didn't already specify a wallet -- so spiders and random visits got a new wallet without there actually being an intent that it is funded. Even wallets that had been funded are spent and abandoned so even a metric for "wallets ever funded", if that metric were provided, would be much too high.
I've seen the "half million" estimate for total users put out there. I've got a feeling that is too optomistic yet but then again every reddit user that gets a microtip has a bitcoin E-wallet, every person that has received a fraction of a bitcoin from those free bitcoin websites has either an E-wallet account or a local client, etc. So I guess if there is no threshold as far as minimum purchasing power to be considered a user, maybe the half million is way to low even. If the metric is to include anyone that has ever sent or received a bitcoin transaction of any size, if that isn't a half million yet I'ld bet it will be very soon.
