Recently searchengineland have posted an article about nofollow links and it's role in google and bing, just read it for yourself http://searchengineland.com/what-social-signals-do-google-bing-really-count-55389 ...and video responce to that topic from Matt Cutts (google), where he speaking about links from social networks role http://www.youtube.com/watch?v=ofhwPC-5Ub4