AIM and ICQ were the big ones back in the day, nothing ever was as big as that.
ICQ was released 1996; Pidgin (as GAIM) was November 1998; Jabber was released in 1999. Those were good times.
AIM was filled with script kiddies using spam to "hack" people's computers. It just crashed the app when they spammed you.
Then stuff like trillium came out which meant you didn't have to use AIM's awful client.
Surely we can agree that no one's experience is universal, and both regional and subcultural differences are likely to have materially shaped our views of the various chat services in the '90s and early '00s...?
Surely we all recognize that our own experience is not The Default Everyone Should Understand?
I suspect part of that is because not everyone had broadband. I was still on dialup years after broadband ISPs were widely available.
Once it did my local social circle used it. Gaming stuff stayed on IRC and ICQ.