For example, I don't remember the last time my team sent an internal email that wasn't a forward from a client. We use Slack. Exclusively. We organize projects around it, sales efforts, everything. It has the async nature of email, the separation of topics like email, and the search power of email. It also means that none of us ever feel like we're out of the loop.
It is hard to overstate how critical this fact is to Email's future. If you look at most of the canonical Internet protocols (SMTP, HTTP, DNS, XMPP, IRC, even BitCoin) - they have this fundamental feature in common. Even though they're part of the application layer of the Internet, they've become fundamental protocols that other technologies rely on. They are part of the infrastructure of the Internet.
I say this is someone who just started using Slack and loves it. I still can't see it replacing email, though.
I actually think these guys are interesting exceptions. Yes, they are still around, but they've largely been eclipsed by proprietary solutions. Why? Why did Twitter storm to success XMPP was ostensibly the same thing but with all of these good qualities (open, federated, decentralised, etc.). Still to this day messaging is a battleground for new entrants touting completely opaque solutions. It seems like there's something more to success than just being open, federated and decentralised.
Edit: GTalk shut off federation for XMPP extensions like multi-user chat and Jingle (Voice-Over-IP).
As for IRC - yes it has been largely eclipsed by proprietary solutions. I chalk this up to the lack of a compelling, truly simple web front-end for it. For instance, Slack seems to imitate a lot of the feature of IRC - it would be easy enough for it to just be IRC underneath, and maybe it is. But as far as I know nobody ever wrote a true web client for IRC that was compelling enough for the mainstream.
I remember people saying that in about 1998. Email killers have been successful, but not at killing email.
To do that to email, you'd have to dismantle DNS.
I'm saddened and proud that every time I read that sentence, I initially think the person is using Slackware.