Knowing that I don't have an answer to your second question, just some ideas of the top of my head.
Not optimizing for engagement at all may help. Driving people who whish to air their oppinions in public to blogs with RSS as a "follow" button may help. Not concentrating everyone and their mother on the same platform may help. Decentralizing the central features of FB to different providers and technolgies will make it harder to concert these propaganda efforts in my oppinion. Not that they will not try, as has been mentionend before this has happened before the internet existed.
It's probably not going to solve itself for sure, but at least the solution is not in the hands of one entity, with perverse incentives to get more engagement to show ads. Besides this, an alternative to FB may not be one technology or service, but several separate ones.
Maybe that's the case for him, but I'm of the oppinion that open protocols will win in the end. You often hear about the network effect of social netorks, however it is important to remember that FB is a subset of the greater network that is the Internet, which is built on open protocols.
I don't get how Mark can live with the monster he has created. He made billions from this, why doesn't he retire and move to a desert island with his wealth. From there he can start to work on an open protocol alternative to FB.
Not optimizing for engagement at all may help. Driving people who whish to air their oppinions in public to blogs with RSS as a "follow" button may help. Not concentrating everyone and their mother on the same platform may help. Decentralizing the central features of FB to different providers and technolgies will make it harder to concert these propaganda efforts in my oppinion. Not that they will not try, as has been mentionend before this has happened before the internet existed.