Matt Cutts addresses links from Twitter and Facebook

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. While Facebook and Twitter links may be treated like any other links, they do still come with things to keep in mind. With Facebook, you have to keep in mind that a lot of profiles are not public, so Google can't crawl it, and it can't assign pagerank on the outgoing links. If the page is public, it might be able to flow pagerank, Matt says. Google uses a similar strategy for ranking Tweets. However most links with Twitter are nofollow.