I heard that 76% of statistics are made up on the spot.
That was supposed to be a conservative estimate, the real figure is probably much more than that.
Over 15 million copies have been sold, even assuming that some people bought more than one copy etc... that's still not even close to the amount of people who actively play Diablo 2 today.
And don't get me started on Diablo 1... virtually no one has that installed anymore.