Agreed. So then what are the most active online communities today that aren't Facebook/Google/Twitter/LinkedIn?

At first cut I think of Telegram, Signal, Discord, Snap, WeChat, Line (East Asia), KakaoTalk (Korea). Maybe HN.

This is what I tell people when we get to this topic. I like to ask 'where do you communicate with other people'

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.

Reddit! It's a sleeping giant.

Google+ is a sleeping giant as well. Not too many users but nice comnunities and amazing features/design.

But I guess they find a way to destroy it.

No! It's email! See my opinion piece above about e-mail the new social network.

Reddit and other social networks could have been implemented using e-mail as a back-end.

You're right! Although email is already more popular than Facebook. ;)

So the user defaults to trusting their email host/server for their privacy, data safety, security, etc? And the users who connect to that user (their data will be sent to the email host as well) will need to put their trust in them as well. Is that the only concern?

Have you thought through what the other positives or potential negatives are of using email?

I disagree, what is unique about Reddit that a smart team of engineers can't reproduce easily?

Like facebook their main advantage is network effect only.

Isn't that a problem then though, if people are using a product for a certain purpose, if it evolves to become something unexpected?

Edit: Read a comment of yours below to see you're talking about email.