In case you are unaware of this: If you begin a personal message to someone here and add people onto it by clicking on the "people" icon on the far right side, all those who are included in this "personal message" will be able to read everybody else's responses. The minute you "add" another person in, it's no longer a "private message" between just the two of you. Any and all messages/responses then become part of and visible by the group at large.
Yes. I had someone add Mark to a private conversation WITHOUT TELLING ME.
CREEPY!