One has the expectation of an almost immediate response, while the other has the expectation to be entirely ignored until someone brings it up in a meeting a month later
In person/phone - I need answer now regardless of what you are doing.
IM - I need an answer as soon as you have a spare second.
Email - I need a response today or tomorrow.
IM is worse at searchability and retention (as implemented where I work.)
Email is worse for synchronous communication collaboration.
I'm the BI guy on my company, and everytime someone new come in it ask me via chat the address from the different BI dashboards and I always ask them to bookmark the pages and don't rely on the chat history because for corporate policies it's auto delete ever so and so.
I tend to communicate with people I know, trust, and like via IM
I tend to communicate with everyone else via email
But IM gives you the pleasure of "replying" with a dot to a previous message you sent to bring it to their attention :)
Personally, I would only use email when I don't need an immediate response. IM is for "I need answers and I need them right now" kinda shit. And if I ain't getting a response fast enough, I have to resort to shudder actually calling someone.
Unless it's an open protocol behind it, it will never completely replace email. There will always be a Slack / Google Chat willing to take some market share and force people to still maintain email accounts.
The main difference for me, is that I refuse to use the IM.
I have email for text, I have an office phone for voice, and I have a cell phone for voice & text.
No. I don't need to keep a separate browser window open just because someone might want to use a different system that does the same thing.
I even set a permanent away message that says "I will never see this, send me an email." But every now and then I still get an email alert that I have an unread IM from 2 weeks ago.
EDIT: I'm guessing the downvotes are from the people who are still waiting for an IM response. 🤷🏻♂️
Imagine having three independent IM type systems that one is expected to keep on top of at the same time. Ugh.
Thankfully, that nightmare is finally over and we are back to one.
I get it but I don't. If I am on the factory floor and need someone to be where I am the IM is the best way. If I call they could be away from their desk or not in a position to get a phone call. If I email it might not get their attention.
For me, I'd rather not set the expectation that people can text me. I'd rather that conversation happen in IM.