read first: official forum post concerning old versions of windows
For what it's worth... even if there is only 1 person still using windows 95, 98 and ME each, that would add up to at least an active population of 6000 people. Given the age of the game, I would dare to say that this is an impressive number, however it is more likely an order bigger pushing the 10000. I don't think there are more than a handful of online games that can still boast these numbers, six years after release.
I would be curious for official numbers, unfortunately these cannot be released for the simple reason they would be compared to WoW's numbers (which, granted, is also dying)