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.