The holidays gave me a chance to re-acquaint myself with World of Warcraft (WoW). I have to say, it's still the single most impressive online game I've ever seen. For my money, it does everything right. It literally *is* for my money, since I'm one of those paying 9 million subscribers.
I'm researching distributed virtual environment (DVE) scalability. As part of that, I was curious about the populations in WoW, both numbers supported per server, and total number of simultaneously active users. "A Measurement Study of Virtual Populations in Massively Multiplayer Online Games" provides a great peek into WoW, based on results from the CensusPlus UI add-in for WoW. Based on that paper, and on CensusPlus results published at http://www.warcraftrealms.com/, it appears:
I've downloaded and added CensusPlus to my characters addins. It's out of date, but still works fine with 2.3. It's fun to see the numbers for your own server.
In summary, congratulations Blizzard on the incredibly successful World of Warcraft, and thank you researchers and add-in developers for giving us insight into player populations!