That little gem showed up in an employee handbook that I know about. I can understand about not visiting Twitter and Facebook while at work but not allowing an employee not to use Linkedin in their off hours in something new to me. Also the fact that they cannot mention where they work or what kind of work the company does............people this is a service company that does not have government contracts nor do they make weapons for 007. The company is run by people who are absolutely paranoid about "reputation" on the internet............so far they are doing good........no one even knows who they are. Internet marketing is not one of their strong points.
So why do companies think they can get away with this rubbish? Because no one challenges them. It was only recently that several states have banned the practice of wanting Facebook passwords or that you have to friend your supervisor so they can keep tabs on you.
I say burn employee handbooks that violate your rights.