The reason I think people are switching to signal, over the less centralized alternatives, is related to ease of entry.
For whatsapp, all you had to do was download it from the app store and give it a phone number, and you're off. Your contacts are already there, everyone is good to go. Signal mirrored that, but removed facebook, so it seems to be a better secure alternative. Things like matrix, XMPP, even IRC require that you pick a server, find your friends, put in some effort.
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