I received this question last week:
Question:Is it possible for other people to be able to read Mineral Rights Forum off line conversations?
Answer: Private messages are not visible to anyone, but the parties included. It works much like an email system. The site owner doesn't even know when, if, or what anyone messages. If a conversation is held in the comment section of your profile, that is visible to friends. If you don't want your friends to see it, don't publish it in discussions or in comments on a group or on your profile. Those are visible to your friends.
Use the email system to keep messages private. You can send anyone a message by clicking on their profile, then clicking "Send a Message" right below their photo. You will have to be their friend before trading messages, but you can include a message in the friend request.