I can absolutely, positively tell you that sites that barely any (or none at all) social 'buzz' are ranking and in quite competitive niches.
I think we are giving Google way too much credit. Not everything that makes sense can be calculated reliably by Google and not everything applies to all sites.
For now, I stand by my fixed month /days penalty for Panda sites. That's the only reasonable explanation I see. Google is ignoring changes, and is in fact lowering traffic even when more top content is added and way less pages are on the site. It makes no sense otherwise.