It's hard to tell, the preferred messenger seems to be different from person to person. I know people who use AIM, YIM, Google Talk, QQ, Meebo, Skype ... There's so many out there that it's hard to say that one is most widely used to teach eachother our native languages.

From time to time I'm asked if I use MSN but personally I use Skype. I'll tell you that using Skype I haven't had trouble finding students, there's a lot of people who use it and more who would be willing to download. However, it does seem the majority of Chinese people use QQ above anything else.

I don't know.. I don't have the latest statistics :P
Well, I guess all depends on what everyone prefers. In my case, i like Skype.