In an interview, both Google and Bing have confirmed that "links shared through Twitter and Facebook have a direct impact on search engine rankings." This has long been suspected by SEOs, but getting this official confirmation is a substantive step forward.
In addition to that revelation, another piece of critical data came via yesterday's announcement:
Danny Sullivan: 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?
Bing: 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.
Google: Yes, we do use it as a signal. It's 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.
Danny Sullivan: 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?
Bing: Yes. We do calculate the authority of someone who tweets. For known public figures or publishers, we do associate them with who they're.
Google: Yes we do compute and use author quality. We don’t know who anyone is in real life 🙂
Danny Sullivan: Do you calculate whether a link should carry more weight depending on the person who tweets it?
Google: 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's currently only used in limited situations in ordinary web search.