@dan, have you seen this study by the DC Federal Reserve on this subject?:http://www.federalreserve.gov/econresdata/feds/2014/files/2014104pap.pdf?p=cite_@inthepixels

As of the beginning of 2014, the Fed concluded that bitcoin had 100,000 unique daily users, and that the number of unique daily users had doubled approximately every eight months over the prior three years.  Extrapolating those numbers to the present puts the current number of unique daily users at about 1.6 million.  

I've written on the additional implications of the Fed's paper here: https://steemit.com/bitcoin/@sean-king/calculating-bitcoin-s-worldwide-domination-day