SuperData Research has a June 2014 update on the state of the U.S. digital games market with some analysis of subscription-based MMOGs, describing how even pay-to-play games have been adding microtransactions to make up for lost subscription numbers (thanks GamesIndustry International). They also provide a chart showing the top 10 subscription games earned combined $2.8 billion last year, with ore than a third of that ($1 billion) going to World of Warcraft, $165 million for The Old Republic, and $104 million for EVE Online. Here's a bit:

The pay-to-play MMO market has been shrinking since 2010, dropping from 30.6 million monthly active subscribers worldwide to 23.4 million this year. To offset the loss of revenue, several key titles have incorporated or switched entirely to a microtransaction-based revenue model. In the past five years, the percentage of revenue for subscription-based MMO generated via additional in-game microtransactions has roughly doubled from 14% for 27%. In addition, the average digital spend, in addition to the monthly fee, has tripled from $16 to $46, worldwide. Despite the overall decline, this category has so far managed to maximize their ability to monetize a shrinking yet loyal player base.