Realtime textual dialog, such as in IRC, ICQ, AIM, or ytalk, is a unique communication form. Allowing for two or more participants to simultaneously be contributing their throughts, reading responses, and calculating replies, it is the ultimate information medium for the attention deficient modern intellectual. It is unique from a voice conversation that might be had in person, over the phone, or via teleconference -- and yet it is distinctly different from email or written mail or other non-realtime modes of interaction. In many ways, it combines the best of both for information exchange.

While there are many unique features to this medium (such as the capacity to easily reference that which was said five minutes ago and immediately apply it to what is currently being said), the most relevant feature that affects the mode of interaction between users of this medium is its asynchronous exchange. Put simply: you don't have to wait for the other guy to shut up to say something.

[a relevant analogy]
In computer network programming, there are two ways that your computer can interact with another computer over the network: the first, called "blocking," involves you waiting for your information to be transmitted to the other party before it can do anything else. Your program just kind of hangs there, idling, waiting for the other party to tell you it got its information alright. You are "blocked" from doing anything else while you wait. The other mode is a bit more tricky to implment nicely, and is called "non-blocking."

In the non-blocking model, you don't have to wait. You just go and march on with whatever your program is going to do and the network politely taps your shoulder when its ready for some more data ("Please, sir, can I have some more?"). In this way, your program is always crunching, never waiting. It is efficient.
[end of the analogy]

In the same way, when you don't have to wait for the other person to finish their point to make a relevant point, and can let them start making a point while you are finishing up with one, you can stay continuously engaged, switching as *you* see fit between reading, writing, and pondering. When I was recently IM'ing with a particularly sharp law student, I found myself engaged fully. I was thinking 100% and alternating between reading and writing in a manner that made me the most efficient. It really struck me afterwards that there was no other medium in which we could have had such a productive conversation, so quickly bouncing ideas off of each other, refuting them, developing others, etc.

However, the same sort of features that make IM excellent for information exchange make it atrocious for social / personal interactions. The lack of emotional context coupled with the need to have deep conversations be synchronous, with spacings and carefully emoted reactions makes it seem utterly impersonal (which it is) and lends itself to wild misinterpretations. I've had whole personal relationships (and even business ones) go awry because of bad IM sessions. Avoid IM for personal communications if at all possible. There are only a handful of people that you'll find yourself able to easily converse with about deep matters in a realtime textual medium.

I think that the funny part about this whole thing is talking with people that use IM-type communication a lot. When two of them talk together, it's not much of an issue, but when a non-IM'er talks with someone who's been IM'ing a lot, they'll find themselves interrupted almost every other word out of their mouth. What they don't realize is that the other person isn't trying to cut them off but is expecting them to continue on without interrupting their speech, simply taking the new input into consideration. It's a multitasking modality of thought for a multitasking society.

Just don't try to multitask your personal converations. =)