If you message me with something like "How are you?" DO NOT IMMEDIATELY SIGN OFF. I get notified when I get messaged and I usually turn to the window within 10 seconds. On rare occasions when I'm in the middle of something, it can take me up to a minute. If you're gone when I flip to your window in less than 20 seconds, don't be surprised when I'm cranky at you later. You've almost certainly disrupted my train of thought and disturbed me and I could forgive you IF you were actually there.
It's like when some jackass puts in a toubleticket marked urgent at 4:30 on Friday. You pick up the ticket at 4:32 and realize immediately you're missing a vital bit of information so you call the guy to find out he left for the weekend already. If it's that urgent, schmuck, file it earlier or stick around for followups. Or don't be surprised when you walk in Monday and find nothing's happened.
If someone messages me with a more friendly message like "Gotta run. Wanna check in later and catch up. See you." I can reset my expectations to plan to talk to at some mutually agreeable time. If they message me, "How are you doing?" and don't reply for 17 hours, I'm cool with that. "Instant" message isn't always mean to be taken literally, but signing off leaves me with a reply unspoken. Then I have to debate about whether it's worth taking to email for a "okay. How are you?" type of response. I wouldn't be comfortable giving any other type of response, given that if they actually cared how I was they'd have stuck around for the response.
I really must poll on this, but I'm too tired and crabby to do it now.