IRC is great. Need help with an advanced math problem (or a basic one)? Try #math on freenode. Need help with an esoteric Python problem (or a basic one)? Try #python on freenode. Need help with ${anything_else}? Try #${anything_else} on freenode. Etc. (If there isn't a freenode channel for your topic, then Googling for "topic IRC channel" will usually turn up something on some other IRC network.)

I can't think of any other tool which solves the same problem as IRC, so it seems doubtful that IRC will be joining the ranks of the dearly departed anytime soon.

Not at all, I love IRC. I hang out on Freenode, consistently in #rit-foss (I'm an RIT student) and I jump around other channels.

Whatever you're interested in, there's most likely a channel on some server which focuses on it, whether it's math, literature, jailbreaks, python programmming, security, torrenting, etc. Here's a list of EFnet channels[0].

[0]: http://www.efnet.org/?module=channels

#startups on freenode has the biggest HN user concentration

IRC is just a protocol, like HTTP.