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

Administered by:

Server stats:

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active users

What do you prefer: Fediverse or fediverse?

You might say it's a proper noun that refers to this specific community that we’re all using to communicate, and thus it should be capitalized. Or maybe you think that this argument is technically true for ‘internet’, and we all mostly don’t bother with capitalization for that either.