Antitrust legislation isn't built around 100 % monopoly, but rather dominant force and cartel-like behavior, especially if they act to suppress growth of potential competitors. This is the core of antitrust jurisprudence: markets only work in presence of vigorous competition, but big incumbents are highly motivated to obtain so much influence that significant competition does not have a real chance to grow and threaten them.

There is a longish recent staff report on the Web of the Congress regarding behavior of Amazon, Apple, Google and Facebook (almost 500 pages) [0]. I actually read most of it. These platforms have already engaged in a lot of stuff that is anti-competitive. And the latest purge of Parler is, among other effects, also a suppression of a potential competitor.

Of course, the tech giants say that they weren't officially motivated by a desire to suppress a competitor, but they acted against potentially competing firms so many times already that one should be wary about their explanations.

[0] https://judiciary.house.gov/uploadedfiles/competition_in_dig...