feature wise, it may not be the best (yahoo and msn have games, whiteboard, other useless crap). however, growing up, i (as well as millions of others) had AOL screen names. since all my friends (and their friends, and their friends) had screen names, it was much easier to communicate with them using AOL instant messenger.
now, it has just stuck. virtually EVERY person i meet in the 18-21 yr old range (which is my age group) has an aim screen name - the same simply cannot be said of any of the other ones.
furthermore, aim uses relatively low amounts of system resources, and deadaim (the specific type of aim i use) has aim logging, which i find to be very useful. for these reasons, i use aim (deadaim).

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