ICQ - 6 digit address. It's 30+ years old at this point. I still use it once in awhile. Sadly, my Compu-Serve account no longer works. :|
What do you use ICQ for? I stopped using it when it became pretty much just a dating service.
I have a few friends from back in ICQs heyday that still use it. It's kind of a running joke. One of us will fire up a client and login, then email the others asking, "where in the hell is everyone" and we'll use it for a week, then it fades again, until a few years later, someone fires up a client, and emails the others asking, "where in the hell is everyone"... ;)
I also like keeping ancient accounts. I've been on some form of the internet since 300bps modems, so I have a number of ancient accounts that I'll pile up, and when I die, they'll be given to my partner, so they can hang on to them, until they pass. Some day, 371 years down the road, a small child will hold a sheet of paper with these accounts on them and wonder what in the hell was wrong with people 400 years prior. lol
edit: That compuserve line wasn't a joke. I still know my username/password from those days.