If refusing to work with a company is impossible, it's a monopoly, right? Otherwise, you are free to respond to the too-powerful company the same way you respond to the too-expensive sandwich.

If a company is using its power to do something harmful, write laws against the harmful thing, irrespective of how much power the company has. Illegalizing shadow profiles of non-users would be a good step forward, illegalizing them only for FB-sized companies would be less good.

If a handful of companies are operating as a monopoly, that's already well-defined as an oligopoly.

I think the only other situation where a company can be too powerful is if they get too much power over the regulators. I expect it's rare to get there without becoming a monopoly or oligopoly, but it's probably not impossible.