They won't understand the question. They have a FB app on their iPhone and facebook.com on their desktop, and they won't understand what you mean.
If they want to message someone "on another service", they'll just use that other service.
And it's ok. I don't have a clue about how the washing powder works, but I know I can clean any clothes with any powder. I am not an electrical engineer, but I know I can buy any random light bulb and I know it will work with any of my lamps. I know next to nothing about logistics, but I am confident that I can use the same address with any random company to have my package or letter delivered to a person.
The world runs and grows on standardization. That's how we progressed from medieval times to industrial revolution! By standardizing tools and measures used in manufacturing! A lot of it we got by accidents of history. Now it needs to be forced against the market. It has to be, and those of us who have a clue need to fight for it.
What's sad is that the new wave of companies try to transplant their greedy and egotistic ideas from the world of software back to the real world. Any light bulb compatible with any lamp? That's about to become a thing of the past, thanks to the IoT bullshit.
Many do complain about it, but can't do much to fix it. People either end up installing 20 different clients to reach those who use other services (imagine installing 20 browsers to read different sites or 20 e-mail clients to use different e-mail accounts), or simply don't communicate with those users when that number grows annoyingly large. This problem affects everyone, and it's pretty apparent even to those who don't visit HN.
If you doubt this, think back to the numerous HN threads where people complain about how they want to keep the Facebook profile separate to their LinkedIn profile. It's unclear why this is different.
I explained it here as well: https://news.ycombinator.com/item?id=10856612
Saying that they are completely unrelated issues appears to ignore a minor detail called implementation(!?)
Almost every mainstream IM client requires you to sign out of one account and into another if you want to maintain separate accounts. That means that if you actually want to use them (ie, get instant messages instantly) you need to be signed into all your accounts at once.
My assumption, not enough people care to sustain a business.
I could jump in and make an integration solution, providing interop between various communication mediums. So will my competitors, and now everyone has 20 interop apps to chose from, and the problem jumps one meta-level up. Or the companies who own the services I try to make talk to one another will decide that I (or my users) break their ToS and retaliate.
Also good luck going with that when everyone's so big on encryption and sandboxing apps, making sure their interop capabilities are nil.
