Heres a good read (The reputable links stands out for me)
Essentially, Matt says Google treats links the same whether they are from Facebook or Twitter, as they would if they were from any other site. It's just an extension of the pagerank formula, where its not the amount of links, but how reputable those links are (the company uses a similar strategy for ranking Tweets themselves in real-time search)
The full story here
http://www.webpronews.com/topnews/20...ok-and-twitter