diberry - 5:16 am on Oct 9, 2012 (gmt 0)

they may well have more of an indirect influence on ranking rather than being a directly used metric

Yes, this. Popularity in social media creates a distinctive link profile, and links are what Google's all about. But Google can't assume a site is quality just because people on Twitter like it. So social probably isn't a factor on its own, but it's probably part of the overall link profile. Or it's a factor, but one that's too weak to create any obvious movement we could track.