A thing to keep in mind that ICQ used to be insanely huge in Russia and neighboring countries. Back in late 90s people would know their own ICQ ID by heart (which are numerical) and exchange them instead of phone numbers. You also could buy vanity 4-5 digit ICQ numbers for anywhere between few hundred dollars to few thousands. Don't know if things changed now, they probably have.
Wow, this led me to discover that I also still know my ID. Before Facebook's rise in Europe around '04-'05 or so ICQ was the online communication channel of choice pretty much everywhere around here, not only in the Eastern part of the continent.
Contact spreadsheets for teens in Germany back then contained name, address, phone number, email address and ICQ number. We didn't even have a MySpace back then ;)