This is a very interesting account of an interview of Google and Bing by Danny Sullivan at Search Engine Land on how social signals, mostly Twitter and Facebook, impact their ranking algorithms. The title of the interview is 'What Social Signals Do Google & Bing Really Count?'.
Following are some of the questions and their answers:
Q1) 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.
Q2) Do you try to calculate the authority of someone who tweets that might be assigned to their Twitter page. Do you try to “know,” if you will, who they are?
Yes. We do calculate the authority of someone who tweets. For known public figures or publishers, we do associate them with who they are. (For example, query for Danny Sullivan)
Yes we do compute and use author quality. We don’t know who anyone is in real life
Q3) 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.
In conclusion, both Google and Bing indeed use some of these signals in their organic web search ranking algorithm. It also is likely that social signals will gain more weight in search ranking systems. Your Twitter data especially plays a role in web search, these days; meaning – getting your page mentioned in tweets by authoritative people can help your ranking in regular search results, to an extent. So, keep yourself updated!