Interesting. I have typically been one to just blurt out question, but have often felt I was violating an etiquette and too type A/ to the point Given the common "hi" and wait strategy from so many others, I thought this might be etiquette. As if to say, sorry to bother you, continue what you are doing and let me know when you can engage for a few questions.
I think of an IM as a faster form of email. I wouldn't send an email that just said "Hi" and wait for a response, so if I IM someone, I'll at least type an opening statement to let them know why I'm contacting them.
My own rule of thumb is that if it's someone I'm work-friends with, I'll be as blunt/casual as can be, but otherwise, I'll open with "Hey [coworker], did we ever get [other coworker]'s signoff on those TPS reports?" followed by a second IM with details if necessary. Being polite, but giving as much information as possible so that a proper answer can be formulated. The less back-and-forth the better, at least if it's work related. Each IM is an interruption, and interruptions should be minimized. If it'll take a bunch of back and forth BS, I'll pick up the phone.
If it's especially non-urgent, I'll say that in my opening IM, so there's no pressure to drop everything and address my stupid question.
The worst is a guy that'll just open with "[my initials]?" and not say anything at all more unless I respond, and I never know how to respond to that. "Yes?" or "What's up?" seem to do the trick. Unfortunately, he's also a part-owner of the company so I can't exactly throw a fit about it.
We can meander and be vague if we're talking about colonizing Mars or what we think Batman eats for breakfast. But we can also go 20 minutes without responding if we're busy and nobody gets offended.
Also, complete sentences and proper capitalization/punctuation/grammar in IMs, please. Punctuation at the very least. We're all adults here. "hey did u add the new chemist to the instrment comp yet tks" by someone that makes double my salary gets as passive-aggressively formal an answer as possible.