Know is much more different than hold, buy, trade and actually use it
Yep, thats true. You can never how many people knows about Bitcoin in this world. Even knowing how many people that are making use of Bitcoin is impossible. The only we can know how many people are making use of Bitcoin is if we allow registration before the use of Bitcoin. With the registration we can be able to link every address to the owner. But not much people will support the idea of giving out their info to make use of Bitcoin.