It's very surprising to me that person-to-person text communication is divided into two completely different and incompatible types of service: short latency (~20 sec, "email") and very short latency (~1 sec, "instant messaging"). A priori, I'd have just imagined that the former would arrive first for technical reasons but then would be consumed by the latter, which has no fundamental technical barriers to being bett…
And IM address is more or less private and restricted - you need to approve other party in order to send you messages.
And this, not the latency, makes the difference.
People are using Skype for example in "email mode": sending messages even when they know that contact is offline - it will be read later anyway.
Yet there are more differences:
1. Uniqueness of address. You may have tons of email addresses. But other party cannot be sure what you are reading any of those. While in IM you have one address in their particular namespace.
2. Consequence of #2: Most of IMs keep conversation as single tape - you know that if you or your correspondent did ever say something you will find it in that particular place - on that tape.
3. More or less guarantied delivery and corresponding UI feedback ("his/her LED is green!"). If you see "message delivered" then it in magnitude of times is more probable that your correspondent have or will read it. With emails it is more like a gamble. How many times we heard advices to check spam folder to find someone's message?