Scoble explains why he stopped using ICQ:
Why? Everytime I start it up I get a flurry of messages. Unlike Twitter IM has an expectation that you’ll answer it sometime soon. But that’s my problem and I’m an outlier. So why did everyone else stop using ICQ? It got too cluttered and stopped being developed. In 1996 it seemed like there was a new feature every few days. At some point after 2001 it stopped seeing radical improvements.
That really wasn't my problem. My problem was a lot simpler: spam requests for chat. As I recall things, ICQ got flooded with that kind of thing - I'd bring up the client, and I'd get tons of pr0n messages fired at me. I get one or two a month like that on skype, and virtually none on AIM. That's the thing that drove me off ICQ.