(a) somewhat tricky to define with legislative-grade rigour, and much more importantly:

(b) only measurable by the very same entities who would be subject to this regulation

Were it not for the problem of measurement, I would agree that it would be a better proxy for "needs regulation" than market cap.

EDIT: On second thought, I don't even think that's true. Imagine a networking app that absolutely dominates a small but highly profitable and societally / strategically important sector, such as medicine or law.

That company could have relatively few users as a % of the population, but still have enough of a warchest to hobble most potential competitors, and to have undue influence in wider society thanks to its control of critical sectors. Going by market cap doesn't measure a company's pervasiveness, but it measures its sheer economic power.