For a detailed and insightful treatment of the evolution of antitrust law in the US, I recommend “Cornered” by Barry Lynn.

The key point is that the interpretation of the law changed in the 80s to a very narrow test of monopoly - whether a company’s dominant position in an industry hurt consumers (ie created higher prices). This ignores, for example, monopolistic behavior that makes it impossible for startups to compete.

The current administration is shaking things up by not limiting itself to this historical definition (though what definition it is using instead is unclear). For the last 40 years, Google would never have been challenged as a monopoly because its products were free to consumers, hence there could be no harm to them, hence no monopoly. Same for Facebook.