Their is no way their are 2 million active-transactors of BTC.  Maybe by some perverse hoarder=user logic like Jill keeps repeating (yet without ever admitting that you might hold WITHOUT using, otherwise the two terms would be synonymous) is using can you come up with number that high but it won't reflect any kind of reflection of actual people engaged in commerce to buy and sell in BTC which might literally be down in the thousands for all we know.

For that matter we don't even have a good characterization of how much of the daily transaction volume is in real commerce and how much is just zero-sum arbitrage and gaming like Satoshi-dice and Mt.Gox other then that were very sure they are the vast majority.

Of flix number the 640K downloads of the client seems most relevant and I'd say that's the upper bound.