I never understood ICQ and never felt the need for it. I had Yahoo for a while because of some forum I used to visit and most were there. I only got MSN messenger because of classmates. This is 1996~2001 or so we're talking about.
My guess is purely because of the activities or who you hang out with. Skype is dumb and retarted as a mass chat service and I only use it for voice-convo while gaming with RL friends or calling those neetpia guys.
IRC is still the best IMO. Simple, fast, clean.