Probably the most important quote from this article is the following:

“While Facebook and Twitter links may be treated like any other links, they do still come with things to keep in mind. For one, with Facebook, you have to keep in mind that a lot of profiles are not public. When a profile is not public, Google can’t crawl it, and it can’t assign pagerank on the outgoing links if it can’t fetch the page to see what the outgoing links are. If the page is public, it might be able to flow pagerank, Matt says. With Twitter, most links are nofollowed anyway.”