How many people use a fake name online?

This question will never be able to be answered 100% correctly without the complete militarization of the human race. To answer this certain assumptions have to be made.

1 - lets assume based on a 66% "adult" population worldwide who use the internet

2 - lets assume out of that 66%, only 50% use the internet in a manner that requires a persona

3 - assume all individuals will use their first name

Math time. 66% of 6 billion is 4 billion people. We know that names are not as unique as people, so, based on the number of J Smiths in my phonebook I can say that out of every 150 people, only 1 will use their real name, the rest must improvise.

So, extremely rough estimate, based on no scientific fact whatsoever, I'd say that approximately 3.9 billion people use a fake identity on the internet