As for emacs, install http://www.emacswiki.org/emacs/TwitteringMode and M-x twit
That said, IRC will never be as popular as Twitter and probably doesn't make for a good business model. But it's all that kruhft said it is.
There's nothing "fragmented" about exchanges on Twitter for quite a while now. If you click on an @message, you can see the entire conversation chain in the sidebar.
Your statement of "fast" confuses me. Minimum roundtrip for a write/reply/read sequence on Twitter is something like a minute. A conversation that would take five minutes on IRC can take several hours on Twitter.
Your definition of "real" is deep and long? Okay then.
>Minimum roundtrip for a write/reply/read sequence on Twitter is something like a minute.
Not in my experience. When using software tied into the API, I've gotten maybe 5-10 seconds lag between entering an @reply and having the client be notified of it. This was while testing, curiously enough, an IRC bot and Tweetdeck.
As for the roundtrip, that's interesting. Perhaps it's just the clients (or the clients I use) that are slow.
I've had way more, and better conversations on G+ and Facebook because it doesn't artificially force you to dumb things down (or possibly worse, resort to unintelligible TwitSpeak).