An interesting article looking at social media sites like Yik Yak and ConnectU and how they died. https://gizmodo.com/why-these-social-networks-failed-so-badly-1836996164
I think there's a lot to be learned here for the Fediverse. I don't have the spoons to distill all the lessons myself, but a few:
1) Big money often precedes death.
2) Neglect leads to toxic user bases.
3) Losing sight of your initial goals alienates your early users, who do more work than you think to make the place run.