The answer seems to be that no one really does. How about this: internet traffic is half-fake and everyone's known it for years, but there's no incentive to actually acknowledge it.
With very few exceptions, no media property big or small, new or old, online or off, can truly tell you how big its audience is. They may have never thought about it that way — after all, we all get as close as we can to what we think is a reasonably accurate estimation, though we have no way of confirming that — but all these numbers are actually good for (maybe) is relative comparisons. What does it really mean when someone says "a million people" saw something? Or ten or a hundred million? We don't know, and neither do you.
A very interesting article on a matter we all know is subject to wild speculation and guesswork but we don't want to discuss it.