>People who do this are generally trying to be polite by not jumping right into the request, like one would in person or on the phone - and that's great! Different people have different reasons but I notice that many people that do a naked "hi" / "hey" are trying to see if the receiver is present for an immediate reply. In other words, the sender deliberately wants a synchronous instead of asynchronous convo. They do…
"Hello" is a pretty bad negotiation protocol for that. At best, the person is online and can initiate synchronous communication (but has to sit around and wait before they can start thinking about your desired context), at worst you get stuck in an endless loop of "Sure, you there?" Furthermore, what if the person wasn't available right now? Do they simply ignore your "Hello?" That would seem more rude. In addition, and ultimately, if the sync communication negotiation fails, you're probably going to have to fall back to async; so you may as well leave the door open to that.
"Hey, just want to chat quickly about that thing, if you're not around I'll ask Alice." That contains the full description of what you want, is concise, and will allow the recipient to not waste their time.
The way I see IM is that every conversation starts with what would be a subject line in an email if a longer conversation is required (whether sync or async). This gets everyone thinking about the context right away and improves your ability to search for that conversation in the log.