http://www.gamespot.com/articles/wow.../1100-6421191/
For the purposes of the report, "pay-to-play" MMOs are considered any games "that earn revenue from subscriptions, expansion packs, and microtransactions-based virtual items and services (e.g. experience boosts, items, mounts, and server transfer fees)."
World of Warcraft Earns $1.041 Billion Revenue in 2013
Still the most successful subscription-based MMO out there.