In a story on CNN, they explain how the social networking sites are becoming a feeding frenzy for identity theft cyber-criminals. According to the CNN Report, the FBI estimates 3200 accounts are hijacked per year. Often the most common way the criminals hijack the accounts is by sending fake e-mails advising there is something wrong with their account. That is old style hijacking. Now a days they send an e-mail with a common name so it appears to be from a friend requesting help, you must just log on to your normal Facebook, or Twitter or other social networking site. When you do the cyber-criminals have your sign in information and are able to discovery what personal information you have provided to the social networking site.
For more information see this article at CNN.