Antitrust law is, I suspect, hopelessly outdated... both the laws and the understanding of monopolies/trusts that are baked into them. These laws were based on 19th-century competition. The problems att were price fixing, predatory pricing (eg price low to kill competition then raise prices), supply chain bottlenecking (how you gonna sell your ore without my trains) ... industrial era trust stuff. The precedents and…
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.