Breaking up a monopoly requires that said monopoly be bad for society, and it's adverse effects can't be fixed via markets due to reasons like size and reach.
Monopoly in its own is neither good nor bad.
Facebook has a monopoly in social network. Is that bad? What is the effect of the monopoly? Is Facebook's monopoly having an adverse effect, or is the adverse effect inherent in the way social networks work (and breaking the monopoly won't help).
Simply being a monopoly is an unreasonable reason to break it up.
You could say that monopolies are inherently bad because they stifle innovation even if they don't lead to higher prices. An unassailable monopolist has no reason to innovate because competitors are effectively locked out.
But it appears to me that there is a psychological bias towards an exceedingly narrow definition of substitution goods and therefore markets, especially in digital technologies.
For instance, Microsoft clearly has an extremely dominant position in PC operating systems. The market for PC operating systems used to be synonymous with personal computing, but that is no longer true. Mobile devices now dominate personal computing.
So Microsoft has lost most of its monopolistic power without ever losing its dominance in the market as it was originally defined by regulators and users.
Similarly, search used to be synonymous with using a web search engine and Google clearly has a monopoly there. But now we search Facebook, Amazon, Netflix, etc for various specific things in different contexts and Google has to pay billions to buy users from Apple.
So my point is that yes, monopolies are inherently bad, but they are also inherently unstable in ways that are not adequately reflected in the current thinking around anti-trust regulation.
Those are exactly the types of questions that I think we have outdated laws regarding.
Outdated laws and outdated economic theory embedded in them. There is only so far that you can extend a railroads, mining and foundries analogy.
I'm sure we could have lively back-and-forth about the impact of these monopolies, but the only legal and legislatively pertinent parts of that discussion are those that can be massaged into an analogy to the late industrial examples that the current legal framework (including, legislation, legal precedents and regulatory bodies) was designed around.
Monopoly in its own is neither good nor bad.
Facebook has a monopoly in social network. Is that bad? What is the effect of the monopoly? Is Facebook's monopoly having an adverse effect, or is the adverse effect inherent in the way social networks work (and breaking the monopoly won't help).
Simply being a monopoly is an unreasonable reason to break it up.