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Also means people can be harassed and the offender can delete the message after they know the person has seen the message (via the read receipt).

Deleting the message will look like admitting a thing done wrong.

/me starts taking screenshots of conversations with others.. ;)

Oh sweet summer child.

Those messages aren't going to be deleted, they're just going to be hidden. They could be un-unsent as easily as they could be unsent.

If you don't trust your friends not to blackmail you, you can't trust them not to just save every message themselves at the time you send it anyway. Even so, "unsending" messages is a meaningless gesture to security because you have no way to verify that anything has been destroyed. Facebooks track record clearly demonstrates that in the absence of evidence, they definitely aren't destroying the data. This feature will be marginally useful for correcting typos, and that's about it.

I doubt the worry was a friend attempting to blackmail.