As of yesterday, both Bing and Google have confirmed (via an excellent interview by Danny Sullivan) that links shared through Twitter and Facebook have a direct impact on rankings (in addition to the positive second-order effects they may have on the link graph). This has long been suspected by SEOs (in fact, many of us posited it was happening as of November of last year following Google + Bing's announcements of partnerships with Twitter), but getting this official confirmation is a substantive step forward.
http://www.seomoz.org/blog/google-bi...-influence-seo