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 thin

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.