Ethernet Access in Office

Nowadays, I guess a lot of company have provided Ethernet access for the ease of communication with the outside world, maybe to the vendor, customer or among colleagues. My company has provided us with Ethernet access facility too. However, due to company policies, our IT department has restricted us to access certain site. Unfortunately, most of the restricted sites are some favourites site that most people using nowadays, such as friendster and facebook. Furthemore, the usage of IM has also been prohibited.

Sometimes, I just wonder, as Management point of view, is this (accessing the above mentioned site/using IM in office hour)really a serious issue? By blocking all these website, does it help to improve the performance of the staff? I rather give them more freedom and evaluate their performance based on the quality of work they have done, but not numbers of time they spent browsing Ethernet.

P/S : I said so because I am also one of the victim of limited access via Ethernet. :P. No more facebook in working hour.