That's why I'm using QuickTopic for the Mafia in my upcoming game and forbidding private messaging. I used to think it added a layer of intrigue, but it's mostly just who can buddy up and lie convincingly in private, 1 on 1. Mafia, to me, is more about lying publically and getting away with it, or convincing people publically, rather than 1 on 1, where it's much easier to sway or voice a discussion and let little things slide without realizing it.