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 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.