I used to work at a company that tried to block everything. Our arm of the company was a web-application creation so you can understand our need to communicate with the outside world. It was important to be able to read blogs, access Facebook, IM our peers, and access whatever new web-based stuff came up.
As new things came up, became popular and were subsequently identified in the mainstream media as "time wasters", the message came down from on-high that it had to be blocked.
I also once worked for an institution that had its own email system and wanted to force all involved to use only that email system. So they blocked as many webmail clients as they could. Nothing else was blocked. Just email. When someone tried to access Gmail, for example, they saw a notice telling them to use the email account given to them by the institution. They were also, quite unhelpfully, told to give all their friends that email address. Unhelpful because they couldn't access their accounts to download address books.
All of these things say to me that there are people, people in authoritative positions who don't understand the nature of the internet. They just don't understand the freedom we've been given and subsequently don't understand the anger we feel at having these freedoms taken away from us.
A business that does not create an atmosphere of trust amongst their staff is actually encouraging staff to find their way around these problems. Really. There will be backlash.
Businesses are in the fairly unique position of being able to choose who becomes part of their organisation. They select their staff. They vet them. If staff members aren't doing their work then they need to look at the cause of the problem. Checking Facebook several times a day is a symptom of a bigger problem. An employee who is challenged, encouraged and excited by their job will complete work before taking on something extra-occupational.
Adults do not want to be treated like children. The removal of freedoms is the mark of despotism but a good business should be more like a hierarchical community.