I dunno, not nothing.
I’ve thought for awhile that if such a dominance came about, as it is now, that someone or something online would become the first virtual equivalent of a POPOS or POPS, a privately owned public open space or privately owned public space.
What hinders it is, what does Facebook owe for? Usually these arragements are tit-for-tat.
Typically the private component gets something from the arbiter of public components, a government.
So, if Facebook owes nothing, sure.
But now… what with Facebook for many years accepting taxpayer subsidies that now exceed one beellyon dollars…
Not much would change, FB would police the same efficient way, but when this occurred, they’d have to put it right the fuck back up.