By the massively higher amount of spam and scams on Facebook, compared to IRL.
I don't step outside on any random day and get immediately blasted in the face by dozens of One Weird Tricks and AI-generated images of Jesus crossed with shrimp.
There’s probably over a million literally deranged people on Facebook.
Even if you only come across a tiny fraction of them, that’s still way more then you could possibly ever encounter in real life, in one physical community, due to simple probability and population density…
And building a thing where you massively increase their reach is a decision, yes.
As an example, it could be Nextdoor-y[1], bound to physical location rather than global. And that's not a stretch either: it's what Facebook was in the early days. They decided to change it, to become what it is now.
[1]: I do not in any way mean to imply Nextdoor is doing things right, just showing that "social network structure" is a decision, not some kind of inevitability.
Because only the most noteworthy fraction of the population are well… noteworthy.
So the only reliable factors to boost the vast majority of the population way above their peers would be money, endorsements, etc…