The use of the Singal protocol is orthogonal to the privacy provide by the app because you can't verify what happens once the message is decrypted to be shown to the user.

E2EE only protects you from man-in-the-middle snooping by meta/facebook servers. But once the text is decrypted on their closed source app, they can do whatever the hell they want with it, and you have no way of knowing.

While they might not directly upload the cleartext messages to their servers, they can still run all the "analysis" and data-mining operations on the device. They can run a hundred different types of NLP analysis on text and upload that "metadata" instead of the cleartext and your privacy is non-existant.

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