A strikingly relevant and simple typology IM users by Danah Boyd. She makes an interesting discussion between IM as a presence versus a communication tool.
I have a round-the-clock presence on AIM, even if frequently idle. I share this round-the-clockness with some of my buddies - people who always appear to be on, although sometimes idle. There are other buddies who pop up whenever they're on their computer (often 9-5). Then, there are those who pop up very occasionally. (...) The thing about members of this latter category is that they *always* want to talk when they come online. This makes sense - they're appearing online only to talk, not to share presence. They are seeing IM as a communication tool first and foremost. (...) Interestingly, it is this group that complains the most about how they can never get anything done when IM is on. I try really hard not to respond in a snarky voice that i can never get anything done when they're on. They get upset when i don't have time to talk, arguing that i shouldn't be online if i don't want to talk. (...) let's go back to the people who come online just to talk. The problem with this group is that they're unintentionally exerting power. They are declaring their free time by logging on and they're assuming that i am signaling the same thing. (...) I don't spend a lot of time conversing on IM (...) The problem with IM is that the always-on'rs have gotten far more comfortable with the technology than those who still see it as a communication tool, not just a desirable presence tool.
Why do I blog this? I exactly feel the same and it's definitely a problem! From a socio-cognitive point of view, it's of tremendous interest. As a matter of fact, mediated communication oblige people to make inference about others' activity or availability. Each of us has rules about how it should work with IM (for instant some thinks that if the conversant does not want to talk he should get offline). And sometimes the mismatch between each others' rules is bad and cause various problem liek frustration, anger or misunderstandings. Another point already raised by Catherine Cramton about email is how do we deal with silence. How can we interprete silence: what means an absence of answer.
About Catherine Cramton, have a glance at "The mutual knowledge problem and its consequences for dispersed collaboration" (.pdf) which appeared in Organization Science in 2001