While technically true, I think calling Mastodon the smallest amongst them is a little indigenous. After all, wasn’t the idea of ActivityPub “protocols, not platforms”?
For me personally, Mastodon opened me to the world of federated social media, and I’m now hosting my own AP-based single-user social media page running GoToSocial. My usage won’t add to Mastodon’s MAU’s count, but 90% of my interactions are with Mastodon users, and I think that’s the beauty of the system.
Exactly that! I want to run a single-user instance, and Mastodon is quite heavy for that, especially when I'll be running it off a cheap VPS.