What Social Signals Do Google & Bing Really Count? by Danny Sullivan at Search Engine Land is a must read by all. Danny interviewed Google and Bing on how social signals, mostly Twitter and Facebook, impact their ranking algorithms.
In short, both Google and Bing do use some of these signals in their organic web search ranking algorithm,
Let me highlight some of the questions and answers:
1) If an article is retweeted or referenced much in Twitter, do you count that as a signal outside of finding any non-nofollowed links that may naturally result from it?
We do look at the social authority of a user. We look at how many people you follow, how many follow you, and this can add a little weight to a listing in regular search results. It carries much more weight in Bing Social Search, where tweets from more authoritative people will flow to the top when best match relevancy is used.
Yes, we do use it as a signal. It is used as a signal in our organic and news rankings. We also use it to enhance our news universal by marking how many people shared an article [NOTE: see the end of this article for more about that].
3) Do you calculate whether a link should carry more weight depending on the person who tweets it?
Yes we do use this as a signal, especially in the Top links section [of Google Realtime Search]. Author authority is independent of PageRank, but it is currently only used in limited situations in ordinary web search.
That bit about authority being a signal that is distinct from PageRank sounds like a big clue to me. The nofollow attributes still stop a link from sending PageRank, as "advertised". But it doesn't mean that a social media mention won't influence the SERP at all.
It's also interesting that they do use the "authority" of the person posting as signal. This would, to some degree at least, help to prevent gaming the social signal with bots and mechanical turks.