The word monopoly unfortunately invites all sorts of pedantry about whether a company is a monopoly (and it goes on endlessly because people can play a game of quibbling about exactly what 'the market' is). Twitter is obviously a powerful player in the social media market, whether or not it is a monopoly. You don't need to be a monopoly to engage in anti-competitive behavior.
Yes, it is often pedantry and not offered in good faith, but in fairness, the legal definition of "monopolization" requires that you either have a monopoly (defined, loosely, as a high-enough marketshare), or, if accused of
"attempted monopolization", that you have a dangerous chance of achieving a monopoly. As you note, that requires defining the "relevant market."
I guess I just find it annoying that people want to quibble about "relevant markets" and that kind of stuff, when they could clear up the more likely confusion, which is basically just vocabulary, instead.
