The replacement for Facebook is likely already being used by many people, perhaps even yourself. You just don't know it yet. ;)
At first cut I think of Telegram, Signal, Discord, Snap, WeChat, Line (East Asia), KakaoTalk (Korea). Maybe HN.
Reddit and other social networks could have been implemented using e-mail as a back-end.
Have you thought through what the other positives or potential negatives are of using email?
But I guess they find a way to destroy it.
Like facebook their main advantage is network effect only.
And most people agree to spend a lot more time in Telegram or even just Whatsapp. If you ask gamers also add Discord, and Slack for a lot of professionals.
Most people I talk to barely use Facebook but accepted it as defacto standard, which probably explains why they can't think about 'replacing' it when in reality it already happened.
Edit: Read a comment of yours below to see you're talking about email.