You could do it the same way you would break up the banks. Just put a tax on companies with more users than e.g. 1M. Instead of a systemic risk tax, make it a data security tax; because once you're past a certain size, the negative externalities caused by a security failure justify the tax.

Yes, we would go back from platforms to protocols and the internet would be good again.

As for when a technology become good? There are a lot of possible criteria, but that doesn't really matter we can come up with a fuzzy cut off and any transparent criteria for it would be preferrable to the current state.

> [W]hat if all social networks limited the number of accounts to 999,999?

That's interesting. It opens the door to the possibility that social networks might pay people to leave, in order to open spaces for more long-term-valuable customers.