(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.)