Good links, thanks. At WWDC Apple was saying that Chinese, Japanese & Korean localisations for iOS Apps are often overlooked, and their data shows they should be the first ones to be added after English, even before localising for the more traditional languages.
I was looking into getting my app translated, but haven't even thought about OAuth providers.
I wouldn't say that QQ is dead, I have definitely seen people use it on their laptops in class at uni. A number of classes also used QQ groups for announcements, Q&A, etc.
I agree that WeChat is more popular, though. Those who have both my QQ and WeChat contacts overwhelmingly go though WeChat.
Of those mentioned in the answers, I think QQ would be your best bet. I have never seen anyone use Renren, but everyone seems to have a QQ number.
Another of those "everyone has it" apps is WeChat, which also provides OAuth: http://open.wechat.com/cgi-bin/newreadtemplate?t=overseas_op...