http://www.gamesindustry.biz/article...y-active-users
32 million active monthly users (unique), 12 million actively daily users (unique), concurrent peak of 3 million players. I don't remember the stats from some of the older F2P games popular in China, but I don't think even those (something like Maple Story or whatnot) can boast anywhere close to this money active users at any point in their lifespan.
Also, Riot is insanely profitable. They got purchased for $400 million a early last year (IIRC) and their estimated daily revenue is between $5-10 million.
I'd say they're doing well for themselves as a F2P game too : P