Those numbers in that chart are misleading. At its peak, WoW had up to 12 millions subscribers monthly,the current 5.6 millions are still monthly subscribers, which is much different than the other millions figures on the chart (total registered account or total sales -- not all of them are on a subscription model, and some like MapleStory are free to play).

To put it in perspective, WoW had 5 expansions, selling at $40-60 each at average of 10millions sales each (and you have to buy some of the previous one too), on top of the subscriptions and it's probably pulling in the minimum of $1-2 billions yearly revenue. It's not even a close call to the 2nd place ...