There's a few going on. None are Facebook, they're unique in their own way.
WT.social seems to take a lot from its wiki roots, such as editing other people's posts. They make money just from donations.
Cocoon goes more personal. It's not about having a thousand friends. They're more a subscription model and can't exploit users.
I'm working at one too. It's something community based. You don't post to your own wall. You post to communities - a residential area, geographic location. We've had gaming groups and schools onboard; it's easier and cheaper to reach out than with Facebook and less intrusive than WhatsApp. It started off fairly serious but we're seeing more Instagram style selfies and Tik Tok videos. We monetize by selling community based services like condominium management.
This is just off the top of my head in 2019. There's probably some stealth mode ones going up in the near future. I wouldn't ve surprised if we see 50 more alternatives by 2025.
I think the important thing is not to make something ad based (which incentivizes addictiveness and privacy violations) and not to focus too much on network effects (which incentivizes monopolistic behaviour).