mastodon.social is one of the many independent Mastodon servers you can use to participate in the fediverse.
The original server operated by the Mastodon gGmbH non-profit

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It's a different thing. E-mail, Matrix, and ActivityPub are all different protocols. Mastodon and Lemmy both exist on the ActivityPub (i.e., the Fediverse).

@reiver ⊼ (Charles) :batman:

@Ghast

I'm not sure if that is a question or not, but in case it is —

Yes, that is correct.