Since Studio Archtype and Cheskin released the first large online trust study in early 1999, I've been interested in trust, and particularly the propagation of credibility through social networks and word of mouth. While BJ Fogg has released research that includes whether or not a friend recommends a site, I have yet to see anything that addresses resonance effects within social networks. If two separate friends recommend a site, I'm more likely to visit. Whether it's word of mouth or RSS feeds, personal recommendations from people I trust are my biggest credibility factor, and I don't see credibility research addressing that as much as it could.
The collected resources in the 'see also' sidebar with Peter's article are a goldmine of recent thinking – I'll have to dig and see if there's much about resonance there. [ia/ – information architecture news]