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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Mysk🇨🇦🇩🇪

Perhaps this flaw is what makes some users think that Signal has a "backdoor" as it is easy for sophisticated attackers to target a victim who's using the Mac app and see their chats. (The same may be also true for the Windows app)