We got a response from a previous post about removing time stamps while Facebooking at work. One upset employer sent a note to our boss saying this was a way for us to show workers how to "steal" from the company.
Spending time on Facebook at work is tricky, I think, but you have to know the culture of your company. If you're in a cubicle/office environment with constant web access (that allows access to sites like Gmail, Hotmail and Facebook) I think it's a given that people will also be on Facebook at work. Walk around our office and you can see that screen up on most computers.
If you work in an environment like this, I don't see anything wrong with occasional visits to Facebook during work. Does this same employer go around and limit personal conversations at a water cooler because they are also "stealing time" from the company? I view Facebook posts as the present-day equivalent to such chatter -- it certainly is at my office. Are smokers not allowed to take breaks because they are also on the clock?
A broader point: I've written about workplace and employment issues for years, and firmly believe that breaks make more productive workers. Granted, I work in an industry that is knowledge-based and values creativity. But taking breaks makes you less stressed. It makes you happy. You work harder. That should make your boss happy, too.