So, I’m not going to claim if it’s a monopoly or not directly, but just provide new angles on this debate…
Twitter may have an x% of market valuations of other companies… but that doesn’t have to be the only metric we base decisions off of. Amazon is more than twitch, so has a bigger evaluation. But obviously that other business doesn’t compete with Twitter.
Twitter might account for (eg) 25% of phone use time (and say fb accounts for another x%). So maybe we define the market as user attention. That might make more sense than a strictly financial approach.
Or maybe, like the OP said, Twitter has 90% of all political actors on it, when the next platform has only 25%. Or maybe it has 75% of all public political discourse. Maybe defining the market as saturation of politicians. After all, it’s way harder to compete if you have to convince all existing politicians to move.
These metrics are harder to gather, but might be more useful to gauge how dominant Twitter is in the political sphere. When people discuss monopoly, they often don’t care about the (vague and interpreted) laws per se, they actually care about how a company has somehow come to be dominant and influential in a negative way.
Throughout American history, antitrust rules has been used against mostly large businesses that were unpopular (politically and colloquially). Business and society had changed a lot, especially with the internet, so if there is a political push, defining monopoly policies against another target and definition is inevitable.