I wrote a comment some years ago at Gamasutra where I mentioned 13m players was the most players WoW could ever hope to achieve, based on growing trends. I predicted back then that by 2010 Blizzard would launch their new MMORPG, because I thought Blizzard would like to catch those leaving WoW in a new hobby, but I was mistaken, they instead launched an expansion.
It took almost 2 years to cross from 11.5m to 12m subscribers. Many dismiss the 600k drop, but take notice that it took almost 2 years to grow 500k, and they lost more than that in 5 months.
The thing is, it is very hard to keep the players interested for so long about the same game, even with expansions. In its 7th year, most of the areas are by now already known by heart, most of the enemies defeated, most of the quests finished, most of the equipment gathered... what else is left for players? A giant paid chatroom with avatars. But you have the disadvantage of not being able to run it in a minimized screen to do something else (as you would with other instant messengers)... Also, people grow, when World of Warcraft launched you might have been a teenager, but now you are an adult and have other obligations like working and taking care of children. And people who were 6 when WoW launched and are now 13 might not be interested in it at all, but instead prefer to hang in Facebook or Twitter, so the install base stalls and even drops.