I wonder how exactly these results figure into the ranking algorithm. How authoritative does a link shared by a "friend" on twitter have to be in the page-rank sense, for instance? Depending on this threshold, it might suddenly become even more important for a website to have a good social media presence (alternatively, spamming on twitter might become more attractive).

Also, I think Bing launched something similar over a year ago, at least from Twitter and Faceboook (see http://www.discoverbing.com/facebook/) .