Technically it isn't, since AIM is "AOL Instant Messenger", where AOL happens to also be an acronym, but it isn't America On-Line Instant Messenger, since then it'd be AOLIM.
I wonder if there's a word for these kinds of nested or multi-layer acronyms. Acronyms in acronyms.
It's like a recursive acronym with a base case, so it terminates.
AOL as an ISP existed in the UK at least, a couple of my friends used it and I had a bunch of the AOL discs for some reason (they came with PC Zone or PC Gamer magazines, maybe)