Comment by Al-Khwarizmi

And apart from the advantages you mention, it was really configurable in terms of the level of involvement and openness you wanted. You could use it exclusively to chat with friends and otherwise be unreachable (as in MSN and their ilk later), or you could be open to chat with strangers. And you could find people by interests, I remember looking for coin collectors from countries whose coins I didn't have :)

Then MSN Messenger displaced it (at least in Spain) and it didn't have even 10% of ICQ's functionality.