It doesn't scale to infinity like email does.
Someone else centralises the data and you have to trust them to keep it safe. Email doesn't have this problem because I can take my emails elsewhere and/or change my address, something I've done plenty of times.
You don't know what they're doing with the data and you don't now who can access it. The same applies with email, except I know who Google are and I know they're "reading" it. I accept this cost. I don't know who "Slow Social" are.
It's a whole new system for elderly (and even not-so-elderly) relatives to get to grips with. They get email today. They use email right now.
It's another website that needs another login for my mum to remember, except she'll just use the same password again (and again), until one day it's breached.
I think growth, innovation, and ideas are important, but I also wish we'd focus on building on and improving the systems we already have.
So I guess I have to ask: why not (better) email? Why not a new email system that's interoperable with the existing system, but offers a better UI focused on sharing stories with each other?