I had ICQ too back in the days, I can't remember my UIN though. But I also used mIRC in Brazilian networks. Never chatted in other networks.
I have mIRC 5.91 installed in a DOSBOX environment running WFWG 3.11 and it still works.
As for ICQ, if I were to use it, I'd do it in one of my VMWare machines (W98 or W2000). But dunno about which ICQ version to use. I got Opera to run with KernelEX, so at least WWW browsing is somewhat possible. Next, I have to set up Outlook Express (on all machines - WFWG3.11, W98 and W2000) to work properly.
I used ICQ a lot, and MSN Messenger too. I was never a fan of Skype or some other messaging programs. When I moved from Windows to MacOS X, in 2010, I used Adium to have both ICQ and Messenger. Then WhatsApp and Facebook Messenger came in and we all know what happened.
EDIT: I actually, logged in through the ICQ web version on my macbook and there are just two old contacts there. Thanks iCloud for saving my UIN passwords hahahahaha
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