A billion installations does not mean a billion running copies. The number is somewhat less. And what about 1 user who's been with the "program" since day 1, that's perhaps 30 installs right there.
I took all this into account when I did my calculation.
So a billion downloads might give rise to more (say, a gazillion) running installations, right? Mind boggling if you think about it.
The download count could be even higher for users that only download each version once, put it on a USB Thumb Drive and then put it onto other computers. I know my download count contribution since 2007 or 2008 would be at least doubled based on that alone if each computer had a unique download.
Would be interesting to know if that 1 billion count is just from what people have downloaded from Piriform.com, if it is the actual download amount would be even more impressive!