Year to date, League of Legends, Crossfire, and Dungeon Fighter Online are each closing in on one billion dollars in revenue. Yes, three free-to-play MMO titles are closing in on $1 Billion USD in a single year. All three exceed World of Warcraft, which is still the most lucrative subscription MMO. That might change once expansion pack revenue from the upcoming Warlords of Draenor is accounted for, however. The total MMO industry, free-to-play or subscription, is estimated at almost $8 Billion USD, from January through September.
This is all according to Gamesbeat and their dissection of a SuperData Research (how is that a real name?!) report on the MMO industry. Of course, there is always the possibility that these products will fall short of that milestone by the time January rolls around, but they are pretty close for nine months in and three to go.
The interesting part is why. The article discusses how easily these games can transition between markets due to how low the barrier to entry is. This is especially true in markets that embrace internet cafes, where the game is already installed. The barrier to entry is creating an account, the customer does not even need to think about payment until they have generated interest in the free content.
The second reason, which is not mentioned in the article, is the curve of revenue by customer type. A flat-fee is some value multiplied by the number of legitimate users you have. You will get at most "X" from a customer, maybe a little less for sales, and zero for pirated copies or customers that simply ignore your content. Subscription games split this off to a recurring income; it is the number of legitimate users for that month, summed over every month. While this will get more money from the most dedicated players, because they are playing longer, this still has a ceiling. Free-to-play and other microtransaction-based models have no ceiling except for all the content you have ever made. This is an unlimited ceiling for consumable content.
This can be good for the consumer or it can be bad, of course. Where a game falls on this spectrum really depends on how it is designed. Also, money is not everything. A game can even be released for free if the developer has a reason to not ignore all claims, whether it was a hobby, tech demo, are art piece. It is up to the player (or their gift giver) to decide what is worth their time or money, and that is okay.