Is there any good way to measure the number of people currently using Bitcoin?
I suppose, to make the question unambiguous, I'd have to form an operational definition of "using". I'd include at least
- buying or offering to buy bitcoins;
- offering to give or accept bitcoins as payment;
- holding bitcoins for their value;
- generating blocks;
- keeping a balance at some bitcoin exchange.
I'd exclude people holding bitcoins because they downloaded the client last year, mined for a few weeks, and then forgot about it---though presumably most of them will remember if they ever hear about it again, so maybe I shouldn't.
I was considering getting a lower bound based on the number of members here who have made a post in the last four months or so, but there are no advanced member search options that I could find. Maybe an admin or someone could scrape the database for that information.