I disagree.
Here's my stance on PM's.
A private message is something sent from one person to another that the sender would like to have seen by the receiver. However, that is based on the presumption that the receiver is willing to do this WITHOUT being asked to do so.
I feel NO obligation to keep anything to myself that I have not said, prior to receiving it, that I will. ASSuming that I will keep a secret without asking me if I will is done at YOUR risk.
People make ASSumptions and then continue merrily along as if the whole world will cater to their wishes. I'm not much of a caterer.
Want me to keep something between us? Ask me if I will BEFORE you tell me or deal with the consequences you have brought by not asking first.