This is a joke metric that I first proudly displayed on Catfood Magazine
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.