ICQ was the messaging service everyone around me used in the late 00s. The two most popular clients were QIP and Miranda. Then VKontakte introduced instant messaging and ICQ was quickly forgotten. Then, around 5 years later, people started gradually switching to Telegram. Today almost everyone I chat with uses Telegram.
The network effects were very localized, depending on which country you were you would have probably used a different service. ICQ faded out pretty quickly around 2002 where I lived, with MSN, Yahoo, and Google, taking over. I know in the States they also had a lot of AOL and iChat.
Indeed, for the UK at least, the ICQ peak was 98-99 wasn't it?
MSN messenger quickly took over after it was released in '99.
I remember hearing at the time that ICQ remained popular in Russia and other Eastern European countries for much longer than it did in Western Europe though.