If anything, WoW census can be used to track trends. If it shows less players, then there's a good chance there's less players. How many less players is still debatable. Also, I don't see you attacking Ielenia with how he "feels" the population is. Cause you know, that's more accurate, AMRIGHT? We know WoW Census isn't accurate, but you make it sound like it's so inaccurate that it might as well be monkeys throwing feces at a button that determines the number of active players.
I was looking for a Vanilla private server and found very surprising population numbers on some of these realms. The servers could be lying, but I was seeing some crazy numbers like 200,000 for a few of them. BTW heard about Elysium?
BTW this is just one method to track activity.