Can you justify why we as a society should want companies to operate at a scale like Google or Facebook?
The simple (and inevitable) solution is to break up Facebook. No need to even get that complicated, at least to start: Facebook, Instagram, WhatsApp.
There was a time when anti monopoly laws were taken seriously. That you are the number one social network should not be used to also become a big player on advertising, selling items, casual games, etc.
Letting companies leverage on a monopoly to gain other markets is dangerous for the economy.
Over the past century, economists came up with a measure of firm concentration called the HH Index [1]. The HH Index is "the sum of the squares of the market shares of the firms within the industry (sometimes limited to the 50 largest firms)". (It's derived from the Simpson ecological diversity index.)
Market share is a measure on customers. The DoJ used the HH Index to measure monopolies because monopoly power was understood to result from excessive market share. The case of non-paying consumers was never legally considered.
TL; DR More than apathy explains the delays in bringing antitrust action against Facebook.
Is there an already-available analogous quantity to market share here? Or do you have thoughts about what might be used?
BTW, it's interesting that the HH index uses sum of squares to measure concentration of the distribution, where something like the Shannon entropy seems more natural -- although, "seems more natural" is subjective I suppose.
I only add the latter because it's separate enough already, and you can now sign up "without" a pre-existing fb account. Is it similar in function to WhatsApp? Yep, but if we split the company along product lines Messenger should be considered a suitable product.