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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Fediverse Report

Here, Zuckerberg's view is that interoperability improves the value of all products. He's positioning Threads here to compete with other products based on the quality of the product and not on exclusivity of content.