Hmm I thought QQ would have made more of a dent (even with the size being relative to some "activity quotient"). In pure numbers I know it wins out and if I am hearing things correctly they have even monetised it in a way others can only dream of: http://www.theregister.co.uk/2010/09/21/chinese_mobile_apps/
This being said tencent QQ is probably the best chat/communications platform I have ever used as long as you can ignore it being part of the Chinese data black hole.