Just saw this post from Yahoo a couple of minutes ago regarding banning Facebook at work - especially at government offices.
Honestly speaking a lot of companies have banned Facebook at work. I have worked in a call center for a year and I have seen people accessing their accounts through their work stations. I'm not gonna come clean but I was part of those that did that. But nonetheless Facebook is now part of our lives.
And even if they ban Facebook in their servers there is a lot of means to access the site without getting detected - trust me, I live with two computer addicts / professionals. Its pretty easy to go through third party links to access Facebook.
Banning Facebook is actually not a way to discipline your employees. And also the employees will be the one to decide if they want to abide by the rules or not.