I check my email twice a day, and HN the same. Anything else that could potentially distract me from what I'm doing gets turned off. I couldn't imagine giving random strangers the ability to intrude into my train of thought whenever they wanted.
It does not, however, alert me in any way as to whether somebody's tried to contact me or not - so it only gets my attention when I want it to, usually because I'm thinking about something and can type responses on autopilot while I do.
I wish there was a way to say, "show me everything from the current conversation alone". Maybe by tracking @replies or something.
I've also heard of Bip (http://bip.t1r.net/), but I've never used it.
I run znc on a small VPS instance that is always running, always up, so I don't have to worry about keeping my connection to an IRC server open. ZNC (the bouncer) keeps me logged in to the IRC server, and I connect to ZNC when I need to be in contact with others.
IRC is actually growing a load. It's pretty widely used. The fun thing is, I don't think anyone really realizes it, and it's still considered 'old stuff' by many.
If you look at the numbers, twitter usage is about the same as one of the tiny IRC networks in terms of volume of messages. Of course it's apples vs frogs comparison, but still... More communication is done on IRC than twitter and all the other "hot" new things.
#startups is good on freenode (If you can stand freenode and their 'ways')
#linode on oftc is great
#googleajaxapis is cool on geekshed
#mibbit is great on irc.mibbit.net :)
I'm on foonetic and subluminal (a new network, about 6 or 7 months old). foonetic is host to the various #xkcd-<whatever> channels, as well as the main #xkcd channel. subluminal is just something some friends of mine put together. the average user count is in the twenties.