> At what scale of massive recurring societal harm (Google, Facebook, etc.) should we confiscate all assets [...] and auction off anything.

At the point where criminal fines and civil damages and penalties exceed the value of assets of the firm.

> disband the company

This is less clear. A difference between a juridical and a natural person is that you can't simply swap out the decision-making apparatus of a natural person. You can, if need be, to a juridical person like a corporation, so its not clear what purpose disbanding the firm (whether assets not needed to be obligations are returned or seized—the former being more analogous to the death penalty) serves.