coinbase alone has:
Users: 2,800,000 (Nov 2015)
so im guessing alot more then that. but when i talk to people i prefer to use low numbers instead of exaggerating numbers out of thin air. so atleast 2.5mill people are using or have used bitcoin in the last couple years.(to be on the safe side)
Where is the mathematicians on this forum? Take this total for the USA and work out what % that is of the total population of the USA and then use that as a indicator for the rest of the world. The problem is
North America have a higher adoption than many other countries and most of these other countries have none of these KYC/AML regulations, so most of the users on those services are not authenticated and
basically a duplication of single users. With the pseudonimity of Bitcoin, we will possibly never get a accurate answer to this question.