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Correct. The only people who can read a message (or hear a voice call) are their intended recipients.

Would it be possible for Facebook to introduce a backdoor later without breaking the Signal protocol, or alternatively "forking" it while keeping compatibility between their clients?

The client has access messages in clear text. A backdoor could easily deliver those messages to a third-party, yes.

This is not something E2E protocols can protect you from. You'd have to audit every piece of firmware and software on your device to verify that's not happening.

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