IRC is indeed "instant" and "messaging", but doesn't belong in this poll. IRC is not an IM client, it is a protocol. It also works totally different than any other IM's; it is totally different in principle. You almost (and I say "almost") are comparing apples with lemons (or how is that expressions again?
). Both are fruit, but both are very different. All the other IM's are brands/types of apples, IRC isn't.
AIM is more popular because it is much older than Yahoo, Google, MSN, etc. So more users started out on that. This said, a lot of AIM users have converted to other IM's (hence the big market shares today of the other IM's). Also if you look at the demographical spread vs. popularity, you'll indeed see that AIM is more popular in USA, and the other IM's are more popular in Europe (especially MSN). AOL (America OnLine), the maker of AIM so to speak, is very widespread and has a very agressive marketing scheme. Back in the days AOL was also almost the only company that provided internet (and everything related). So all this explains a bit the widepsread use of AIM.
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