NetworkWorld has an interesting article today on the perils of social networking. The article focuses on the risk of employees transmitting confidential data. However, it's actually worse than that. There are also risks of malware infection via spam and other social engineering tactics. Twitter is notorious for its lax security. See my post, Twitter is Dead.
Blocking social networks completely is not the answer just as disconnecting from the Internet was not the answer in the 90's. Facebook, Twitter, and LinkedIn, among others can be powerful marketing and sales tools.
The answer is "IT Security 2.0" tools that can monitor these and hundreds of other web 2.0 applications to block incoming malware and outgoing confidential documents.