This will be a pretty stupid move to join the forum, Facebook could track you down very easily if you want to remain anonymous then don't join it.
Actually the number of people doesn't matter. all we need is people using it and the market reflect how many users are there. And these wont reflect the real numbers too
There is just no way to estimate it accurately. As bitcoin is created for anonymity. And many people wont show up on these forums.
Bitcoin is created to fuck banks and stop slavery.
Anonymity is a collateral feature that HAD to be implemented.
On top of that, people would only know that you have some Bitcoin. So?
And on top of the top of that, NSA KNOWS you have something to do with Bitcoin when you launch your wallet and connect to a node
They control each and every connection, so they see from home at n.XX Dumb Street, where Genius Guy lives, somebody just opened a Bitcoin wallet, because you communicated with a Bitcoin node.