A Mastodon instance is used by, at best, a few hundred to low thousands of people, and is going to be small and relatively obscure

Both Gab and Truth Social are Mastodon instances (albeit not federated, though if they ever enabled federation they'd be immediately blocked by a majority of instances due to a combination of anti-corp and anti-right sentiments). Gab was actually the largest Mastodon instance for a good while (unsure about currently) - if you see any Mastodon clients that have negative reviews about not connecting to the largest Mastodon instance, that's what they're referring to (several clients blacklisted Gab at the client level).