I think we're still missing an "open social" closed social network. Something like old-Facebook where you can post to an intimate audience of friends and family, and your feed isn't stuffed full of ads and influencers. Just a little private windows into your friends' lives.
That feels like something that could displace other social media in a way that's difficult for for-profit businesses to replicate since it goes against every product manager's instinct to leave engagement on the table, and would stand in stark contrast to the current social media landscape.