It seems perfectly reasonable to set a (somewhat subjective) threshold and start treating companies that exceed it (in some way - e.g. number of users, % of cov

It seems perfectly reasonable to set a (somewhat subjective) threshold and start treating companies that exceed it (in some way - e.g. number of users, % of coverage, revenue) as "public services" and controlling them more tightly. The precedent is well established (e.g. GDPR, breakup of Standard Oil, ...).