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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jwz

@evan @bignose @evanprodromou My technique was brute-force simple. Get the list of every Instagram account I followed. Convert those to [@]name[@]threads.net. 41% were not 404. Of all the rest, none succeeded at federating when I pasted them into the search field on mastodon.social. (Though I do follow one or two other threads accounts, such as POTUS, so it's not that they don't work at all on this instance.)