In my opinion, it's more of a security risk because of the social stupidity factor. People put all kinds of personal information on their Facebook pages that can often be used to guess their user names and passwords for secure sites. Sometimes they are even more helpful by listing some of those sites such as the school they attend, the bank they use, etc.
That being said, I block Facebook and Myspace and other similar sites on my network simply because people spend WAY too much time there instead of doing the actual work they are getting paid for.