I guess there must be a Chinese Bitcointalk equivalent, so add that number in too. It would be a massive underestimation, but I fear that most other numbers would be overestimations.
2 million users/holders would be my ball park guess.
There has been some million transactions on the blockchain in all of history, we know that many people do hundreds or even thousands of transactions and some websites will do hundreds of transactions per day, so it seems fairly impossible that there is even 2 million users. Maybe a little less.
If what you are saying about total number of transaction being only a few million than the number of people using bit coin can't really be even a million. It's amazing how much publicity it has received though.