101919 here. My account got hacked in 1999 and so it's been 25 years since I've been able to login, and yet I remember my UIN the way most people remember their first phone number.
My friend at the time, 101449 was in the first 1500 users; I was late to the game and joined later that afternoon.
I met three people by randomly searching by criteria who are still friends to this day. Erin from Niagara Falls was the first person I had an online voice conversation with. I went to visit Evgenia from Ukraine in St. Petersberg in 2004. I helped Juliana in Sao Paulo practice her English, and went to visit 13 years later.
The thing that was most amazing about the ICQ client was that it implemented a store-and-forward message queue, which was especially important in the dial-up era. That is, someone could send you messages while you were offline, and you'd get them when you logged in.
ICQ was also the first to implement Away, Invisible and DND modes.
While I'm sure that we edit out all of the rough edges in our rose colored memories, ICQ really was the first IM network that felt like a projection of our selves online. NNTP and IRC had their place, but they were radically different interaction modalities. ICQ got a lot of things right that we now take for granted, in an era where techno-optimism wasn't ironic.