There's a larger concept that I think you are overlooking. Real identity vs Hidden Identity (or Psuedo Identity). This is the fundamental difference between Facebook and Myspace. People tend to act very differently when they are using their own identity (portrait of themselves, real name, etc), sometimes negating the need to even have a "karma" type system.
Also, if your "social" app requires friend requests/acceptance, as most "social" apps do, you further negate the need for a trust system. This is because the mere existence of friending/friend clusters are used in place of a trust system. You're implicitly trusting everyone you "friend".
That's a good point. I was originally thinking more along the lines of a flat user model (e.g. any social news site), in which everyone contributes to the whole, rather than a relationship/network-based model, in which certain users interact with other users, but not necessarily the whole (e.g. Facebook, Myspace).
Yeah, I think there's a disconnect in our vocabulary here. I see them as User Generated News Sites which is a different mentality than Social Networking, which I assumed when I saw "social web app".
Also, if your "social" app requires friend requests/acceptance, as most "social" apps do, you further negate the need for a trust system. This is because the mere existence of friending/friend clusters are used in place of a trust system. You're implicitly trusting everyone you "friend".