I can anecdotally disagree with that...in fact, the only reason I got a Twitter thingy after resisting for about two years is because the crowd in my IRC hangouts incessantly refer to "tweets."

Twitter is certainly not a replacement for IRC in these circles either, though, merely an extra tool. It is not well-suited for it, not least because support for anything like real-time conversations between multiple people is almost completely absent -- which I think the correct design decision.

Bizarrely, some of the non-IRC people are trying to sell Twitter as some kind of a chatroom, see for example this recent TIME Magazine article:

http://www.time.com/time/business/article/0,8599,1902604,00.....

Edit: tautology.