Blocking

Blocking certain websites from within your company network seems like a good idea. There are certainly some that pose a direct threat to network security, and they should be kept at bay. The trouble with taking these steps is that it quickly becomes a power issue, and it is easily abused, or at least taken too far. When someone notices that a few employees spend five minutes per day checking personal email on sites like GMail or Yahoo, someone in power cranks the handle and network access is locked down further. When you squeeze like that, some people take it on themselves to fight back, which is the start of a battle nobody wants.

Mokalus of Borg

PS - This is all harder than it seems, of course.
PPS - Our internet policy at work is notoriously limited.