PMs stood for private messages if I remember correctly? But then the standard term became DMs: direct messages. It seems that in many ways, for many years now, we've been subtly primed to accept the stripping of our privacy.
Discussion
I don't think they ever were necessarily more private. If anything, it's probably just a more accurate term.
Right. I don't think they were ever private. The shift in terminology has always seemed interesting to me, though. I'm probably overthinking it.
It used to just be messages.
Instant messages was also a common term before PMs
Facebook named their messages "private messages" to trick people into thinking they were private. They never were.