This is a joke metric that I first proudly displayed on
Catfood Magazine back in
2007 1997 (it's broken on the archive of the site). Everyone had a hit counter back then, but as far as I know we were the first site with a non-hit counter.
The dirty secret was that the counter just showed the world population. The readership was a rounding error.
My new count of non-visitors uses the US Census Bureau's
world population estimate and subtracts unique visitors from the
Google Analytics API. The count is cached for an hour so it doesn't slow the page down too much.