Outside the United States, very few people were on it. When I signed up to play some video games with some US friends from a forum, I remember having to Google a valid US address and it's matching ZIP code as it wouldn't even let me sign up without one.
MSN was more popular in western Europe and as I understand it ICQ (eventually federated and shared some tech, but not the same network) was the top early IM in eastern Europe. Can't comment on the rest of the world at that time but I doubt the sign-up form was any more accepting of Korean addresses for example.
Imagine if the global phone network was balkanized like messaging networks...
TBF, we asked them to let us use the protocol, or make it part of IMPP, or get actively involved in IMPP.
afaik AIM was a derivative of ICQ, but yea those were the good times. today i know more ppl that won't touch whatsapp then there were w/o icq numbers back than, probably the closest we've got to a ubiquitous instant messanging. i still remember my icq# althou i havent used it in like 15y and have burned throu like five phone numbers since.
However, this breaking down of the protocol to small components is the strength of the protocol, not a weakness.
On a related note, AOL should've open sourced and federated AIM. Everyone in the late 90's, early 2000's was on it.