I've always believed that this was one of the primary motivations for Facebook separating Messenger out into its own app: it feels separate and therefore more private, without actually being either.
The content of a post with privacy controls restricted to just one other person is functionally the same as sending a message to that person on Messenger — only the UI is different. But, to the average person, it feels completely different because it looks like texting.
The content of a post with privacy controls restricted to just one other person is functionally the same as sending a message to that person on Messenger — only the UI is different. But, to the average person, it feels completely different because it looks like texting.