Yes and no. Twitter is dominant for their slice of social media, as demonstrated by the fact that a lot of the people who think about leaving don't see viable alternatives. But I agree that US anti-trust law is so hands off at this point that there's no chance the FTC would do anything substantive here.

If you define Twitter’s market as “sites that do things very similarly to Twitter,” I agree they look dominant. I doubt U.S. courts would agree with such a narrow definition, though.

Twitter itself doesn’t seem to take that view of its own market, given that Facebook and Instagram are first on the block list—both products of Meta, a competitor with far more eyeballs and revenue than Twitter.

The FTC themselves say "a product market in an antitrust investigation consists of all goods or services that buyers view as close substitutes". So I don't think it's "do things very similarly to Twitter" as much as it is "serves the same need as Twitter". And as I said, looking at the discussions around leaving Twitter provides plenty of evidence that close substitutes are not available in the view of users.

Twitter is already actively regulated by the FCC and operating under a fairly strict regime.

Twitter is under a consent decree by the FTC related to user privacy, not market competition.

True, but that means that they are not too insignificant to be regulated.

You can browse all the companies that the FTC has consent decrees with. You don’t have to be large. The existence of a consent decree says nothing about their market position.

Twitter has very considerable market power. That's the term in law, not "dominant force on the Web" or monopoly. No-one can use market power to extend or preserve market power. That's precisely what's happened.