Uh... is it just me or would PMs really just create another level to the intrigue of the game?
You shouldn't consider anything you receive in a PM from another player to be automatically true any more than you consider anything posted in thread to be automatically true. There is nothing preventing the mafia from sending PMs to townies to wig them out. Even if a seer is able to confirm an individual's identity, that individual has no reason to trust such a revelation received in PM. After all, the mafia know everyone who's not a part of their faction too. Unless they have reason to believe that individual is acting pro-town, they have no reason to give credence to any information received in a PM.
Alice and Bob are mafia; Charlie is a vanilla townie. On night 3, Bob sends a PM telling Charlie 'Hey I know you're not scum because my Seer Powers told me so. Also you can trust Alice.' Charlie reads through the posts from the first couple of days. Alice doesn't really seem to do much but Bob seems fairly town, pressuring people, asking good questions.
Should Charlie trust this information or not?