Should employees be allowed to surf facebook and other social network at the office?

A FEW months ago, I was asked what HR's perspective should be on the usage of Facebook and social networks at the office. Some companies have built firewalls to bar their employees from Facebook and Twitter usage while others don't but get frustrated watching employees on various social networks throughout the day.

Many fear productivity in the workplace has been compromised with the advent of these social networks. Yet another camp claims that social networks increase productivity. So, the bigger question is: Do social networks increase or decrease productivity on the job?