The Work Clinic has an interesting post today about the debate over how employers should handle the use of Facebook and other social networking sites in the workplace, since some employees may be spending hours a day on them. The post rightly points out that banning such sites sends a message to employees that they aren't trusted.
In addition to that, though, a ban is the lazy manager's way out. If someone is spending ridiculous amounts of time playing online, a manager is going to see it reflected in that person's work product and/or productivity. That's the bigger issue that should be addressed, not the specifics of how the person is spending the time they're wasting. There's no way to ban all the possible ways people can waste time (chatting to coworkers, daydreaming, taking excessive smoking breaks, zoning out while staring at the computer screen, etc.); the more effective option is to pay attention to work output and speak up when it's not what it should be.