For the large part, such a trust network already exists with Facebook. Yes, it's not perfect, and yes, it probably can be better, but when you want some form of trust it's usually very powerful to use Facebook Connect to allow a user to see another user's FB profile.

I've been recently involved in creating a site for sports meetups (like, say, for a casual game of tennis), and FB Connect came up pretty early on in our discussions as a way of getting people to trust each other.