Just go online and use Meebo. It has everything on it: AIM, Yahoo, MSN, and ICQ.

Every kid I know uses AIM.

MSN may be for the entire English speaking population. But I would say in the US, AIM is by far the most popular general chat tool (at least among teens, and young adults), followed by Skype and MSN.

For general chatting MSN is likely the most popular. There are other ones such as yahoo and AIM that are used a bit, but not nearly as much. Skype is kind of a newer tool, for voice chatting it's great. Within a community like this which has a focus on language exchange and thus oral speaking skills, Skype would be rather popular. Some people just use Facebooks built in IM feature, though it's a bit clumbsy for me.