If you write something on your note, which Mr. Facebook doesn't like he takes the liberty to alter the note before handing it to your friend.
If I write “A” is it possible that “B” arrives?
Or are you talking about censorship, so if “A” is unlawful, then it does not arrive at all?
[1] https://www.google.com/search?q=facebook+censoring+messages
[2] https://linustechtips.com/topic/912936-facebook-private-chat...
If you write something on your note, which Mr. Facebook doesn't like he takes the liberty to alter the note before handing it to your friend.