ICQ ; It was the first instant messenger, the technology could have adopted voice (and not get disrupted by Skype) and mobile (and not get disrupted by whatsapp) and group chat (and not get disrupted by slack/discord). But they didn't even try and put up a fight.
Side note that not remembering it has nothing to do with memory deterioration. Neurons that fire together wire together: if you haven’t used that particular piece of information in a while, your brain gradually clears out links to it to make room for stuff you are currently referencing. So not remembering it is really more a demonstration of how much ICQ use has deteriorated. :)
I know. I probably hadn't thought of that number even once in the preceeding fifteen years, and yet it was still there. I had a stroke in 2020, prior to which random detritus like my ICQ number stuck around, along with genuinely useful things like a dozen or two poems that I love. Some of those I've tried to re-learn, and they no longer stick for very long. It is what it is, and I'm not objectively badly off - I'm still able to do a cognitively-demanding job, for instance - but I miss those little things.
The last time ICQ was mentioned on HN I could still remember my ICQ number. It's a benchmark for how much my memory has deteriorated in the last five years. I do still remember it fondly, though.