They have a worldwide monopoly. If countries can't enforce their law on these platforms they will likely ban them on their territory. No Nation want to forgo their sovereignty to US corporations and delegate to them what is fair speech. Expect their algorithms to be under scrutiny as well.
I think US megacorps have more malleable ethics than you are giving them credit for. these corps already have the ability to federated their services, and don't seem to have an issue with meeting the demands of less-than-savory governments like China's
