[gregoriol]

Chat is natively async: you don't need to know if people are there to ask a question, they might answer at any point in the next few minutes or hours.

If you do post just "hi" in a message, your question should come within seconds after.

[Chris2048]

true, but the same is true for email.

I use IM as more informal, (AND/)OR conversations with likely back-and-forth, that may require the other person to be present to be of use.

[gregoriol]

The risk is that by the time they get back to the chat and answer your "Hi", you might be gone and so their "Hello" answer might be useless to sync you both. Examples others provide to request a timeframe to discuss the subject would be much more efficient

[smeej]

So ask them for a time they're available to do that. Don't just ping them and hope to get lucky. Even if they have time to send you a "Hi" back, that doesn't mean they have time to talk to you, so you're going to have to ask that anyway.

[vundercind]

“Hey—have you got some time to chat about Fizz today? [Optional second sentence indicating level of urgency in your part]”

If they don’t get back immediately, you can safely assume “right now” is not a time they have. If they actually don’t know shit about Fizz and you need to bug someone else, they can communicate that when convenient without needing you at your keyboard when they do it, because at least they know WTF you want to chat about.

There is no benefit to just “hello.”