I actually built this once, a long time ago for a very bizarre social network project. I visualised it as a mesh where individuals were the points where the threads met, and as someone's trust level rose, it would pull up the trust levels of those directly connected, and to a lesser degree those connected to them - picture a trawler fishing net and lifting one of the points where the threads meet. Similarly, a user whose trust lowered over time would pull their connections down with them. Sadly I never got to see it at the scale it needed to become useful as the project's funding went sideways.