I won't enter the legal aspects, as the vary from country to country and depend a lot on the company policy.
Nowadays, some Internet, let's say "social stuff" has replaced traditional communication ways like phone or letters. I may drop an e-mail instead of sending a fax or may send a message (via Facebook, mail, LinkedIn, MSN Messenger or whatever).
Would you agree a company policy forbidding phone calls would be ethically correct? I don't think so. There's no way to inforce professionalism to someone that isn't a professional. If I want to waste my time, I don't need FB or adult sites, I can just stand looking at flies flying.
So companies have to detect people being unprofessional or wasting times, not chasing after that efficient worker that sent an e-mail to his/her friends telling them he/she is late because he/she is working.
No one, and with no one I mean no one can work all day without a break, that's improductive, so I think some way of entertainment is even good for productivity.
But you can keep being dinosaurs, banning everything that can be assumed to joy or happines, and getting people into cubicles without windows so they will work and work.
Cheers,
Dian