Good idea. It actually has been implemented already. You are also not very specific IMHO.

How do I know that after a million users you won't turn into another Mark Zuckerberg like czar?

I'd rather stick with the other attempts: Mastodon, Ello, Diaspora. They all offer good levels of privacy.

Haha I hope I don’t turn into one! It’s hard to get it all on a page and still be a little concise. Would love to answer any questions. There certainly are many options, for the ones you mentioned federated or decentralized I feel like the concept while really cool isn’t as accessible as just a website. Other options are also too niche. I think Facebook did a couple things well one of which was have a diverse user base.

I'm concerned that donations would not be enough to pay for moderation of the site when it becomes mainstream and you'd get all kinds of abuse, inappropriate/illegal content, etc.

Abuse and illegal content is a huge worry of mine. Maintaining a moderation working group is a concern at scale for sure. My aim is to adopt a volunteer moderation program similar to reddit.