It's clearly not. When they state user numbers, they only ever state active user numbers. And it's growing like a bat out of hell.

Let that sink in for a minute. Almost 600 million people use Facebook every single month. That's not the number that's signed up, and left. It's the number that is still actively using Facebook.

The derivative of that ratio is far more interesting than the ratio itself.

There is no way to possibly find this out.

What you 'personally think' this number is should suffice.