Most employers can't get enough 'inside' information on their employees. The temptation to spy on employee's social media sites (Twitter, Myspace, Facebook) is great.
Don't do it!
There's a case coming before a New Jersey court later this month. Employees at a local restaurant created a password-protected myspace chat room, where they could (on their own time) comment and vent on the issues of the day.
The owner apparently got an employee to spill the password, and found out the 'inside information'.
Whether this case has legal merit is another argument for another day.
But ethically and realistically, don't spy. If worktime is wasted on social media, have your IT person prevent employees from using them. But spying can only get you into trouble.