Clarification for those debating the subscription numbers. I am a FF fan and dislike WoW (just not my cup of tea.)
http://users.telenet.be/mmodata/Charts/Subs-1.png
http://users.telenet.be/mmodata/Charts/Subs-2.png
The verdict? WoW holds the most active subscriptions in the MMO world. However, one thing can be said about XI which is consistency. That consistency only dropped off with the release of EQ2 and steadily declined for a 2-year time period. Note that this does not correlate with WoW's numbers. This indicates that the FF franchise and the WoW franchise do not detract from one another.