There's a site that is using some online scouring algorithms to gauge sub numbers, activity, sentiment etc, called mmo population and it has ff14 being very stable and slowly increasing which imo is a sign of a healthy game, unlike WoW's peaks that vanish within a few months like the disaster that is SL dropping sub numbers incredibly fast which is easily seen inagame by the amount of people whose friend's list goes from many people to 0-2

They are using the same algorithm for all games so I am inclined that even if their numbers arent 100% accurate, their gauge and comparisons are pretty accurate imo.