*shrug* Dunno.... seems common sense to me....
Anything I may say in a PM could possibly be repeated, especially if I don't know the person. So, I'm very careful about what I say until some level of trust has been established. This is regardless of who initiates the PM.
IF I establish some sort of rapport with someone and we establish a friendly level of trust, I'll be more willing to share more personal information and generally take the friendship offline to e-mail, IM and/or phone numbers. I realize that, because people are people, some things may be repeated, even unintentionally. I'm still careful with what I share, but enjoy/don't mind sharing more information about myself.
BEFORE I share ANYTHING that I would really consider personal, private and secret, I make absolutely certain that I trust that person with such important information. Even then, it's no guarantee - but that's the way life is, c'est pas?
Attack me, completely unprovoked and yah, I'm gonna give Paw a heads up in case the person is certifiable and doing this to others, but I'm also going to just block that person from being able to send me more PM's. Then move on and forget about it.
If it comes up that this person is pulling the same crap on others, yah, I'm gonna expose 'em. That means publicly or privately.
But I never get any PMs, so it's never been a problem.
It just seems real simple to me, "always act as if anything you do or say could be shared".
This is how I decided who to trust and who not to trust with who I was or any other personal information when I started at WayDale as RocknHardPlace, then insurgent, and WaywardWayfer. Obviously, and very thankfully, I chose to trust the right people because my identity was never compromised by anyone by myself and the WAYGB.