According to this article in Washington Post, SSNs can be guessed easily using publicly known information: http://www.washingtonpost.com/wp-dyn/content/article/2009/07/06/AR2009070602955.html. It is also covered inhis article from Fast Company explaining how information users post on Facebook can be used to reverse engineer their SSN: http://www.fastcompany.com/blog/chris-dannen/techwatch/facebook-new-algorithm-can-guess-your-ssn