A closed and self hosted family social network without the marketing momentum would be great. Self hosting removes the need for marketing and data sharing shenanigans that most companies require for growth.
It's also not something most people are going to find out about, because, yeah, probably more-or-less impossible to monetize, and therefore ain't gonna be advertised.
I've also been involved in some experimentation with private Slack groups, but I can't say it's a great option. For people who don't want to be checking their smartphone constantly, it feels like this wild uncontrollable deluge of communication. And people who don't want to install an app on their smartphone quickly stop bothering to go to the website, either.
I have 3 separate group chats I share stuff with: 1. Family 2. friends from uni and 3. colleagues from old work.