This may sound like an extreme generalization, but seriously, from what I've seen, mostly (and by that I mean like 99.9%) of AIMers are Americans. Since 99.9% of my friends are not American, I find that most people use MSN.
(I'm basing this on my own limited encounters with people, mind you.)
Oh, and Trillian really can't be considered an alternative. It doesn't have the MAIN customization features of MSN (the special emoticons, winks, display pics, webcamming, etc...)
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A Resident Dweller in... The Empyrean
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