EMail has it's problems. ~~Abusing~~ Using it for social media updates doesn't sound like something Email can solve well.
It's not necessarily fast, delivery may take only 1 second but I've also had email take days until it was delivered successfully.
Reliability is meh, it has some resilience to services being offline for a bit, so I'll give you that.
The near-zero cost is also not quite true. It's only true if you sell your data to the provider of your choice, gmail or yahoo or AOL. If you want privacy on email, you'll have to pay for it, otherwise you pay by other means first.
There is a chat app that uses email to function, it also features encryption but IIRC the userbase is very small and last i tried it for testing, it did quite spam my inbox.
(1) Everyone's identity is their email.
(2) The person whose email you're typing in has already signed in to the service (or meta-service if it's a federation), and agrees to communicate with you.
If you squint just right, this exact thing already exists; it's called email, and adding to friends means adding to a mail list.
I wonder why there were no (known to me) attempts to actually use the email infrastructure (fast, reliable, ubiquitous, near zero-cost) to distribute social network updates. It could use a custom pretty frontend app that makes posting or reading updates easier than e.g. gmail.
(As a bonus point, the actual emails can be reasonably human-friendly, as an extra archive of all of your updates.)