> How is this determined?
That's my personal judgement.
The actionable response would be progressive taxation on corporate profits which would stop companies getting quite so large, and force (or strongly encourage) companies that wanted to continue growing beyond a certain point to split (which would have the effect of spreading control/power over the economy around).
> The funny thing is Facebook (or Meta I guess) could fall of the face of the earth tomorrow and humanity would not really be impacted other than temporarily losing WhatsApp groups and having to migrate to a different chat program.
Isn't that exactly the problem though. So much of societies resources going towards something that isn't ultimately that important. The opportunity cost here is huge.