I've noticed it's more of a geographical thing. Almost everyone I know internationally is on MSN. YIM seems to be big in India. AIM is big in America, especially among those who grew up around the internet.
AIM is a very useful tool, but kind of unprofessional in my opinion. Just reminds me of gradeschool and talking to my e-friends about Diablo II. Unfortunately there is no industry standard that has quite this much reach as AIM does.