> I know a lot of Facebook clones have come and gone.
There's three problems with most of them 1, micro-management by the owners that destroys communities before they even form and 2, people want to be overnight Zuckerbergs and when that doesn't happen they fold up shop, they don't stick with it. And that leads to an effect where people are unwilling to invest (time and energy) in a new site that might disappear overnight. And 3 allowing 'free speech' means your site inevitably dissolves into a far-right nazi cesspit, so you need strong moderation, but not so strong you destroy your site.
> My question is why hasn't anybody succeeded?
It very much depends on what you mean by 'succeeded'. I run a network of Facebook clones (although functionally much better) and I'd say I do ok, but more importantly I help people make friends and connect. My sites have produced irl friendships, relationships, even a few marriages!
To me that's success, not everything needs to be 'planet scale' and in fact with social networks, you literally CAN'T be a 'community' at that size.
The future is federated networks, everyone 'owns' their own twitter/facebook and can interact with everyone else, like email but social.