>> Can any government, acting alone, shut down or meaningfully regulate these Internet giants?
I don’t know, but government intervention is not required to shut them down.
They are businesses.
To shut them down, it’s sufficient that the masses simply stop patronizing them.
With Google that would almost require to stop patronizing the Internet given that almost all of their revenue comes from PPC advertisements on other people’s web pages and servers advertising yet other people’s companies.
At it’s heart Google is an advertisement firm using their search engine to drive placement of those adverts.
That, or a few well placed asteroid strikes ... it could happen!