if you have a certain amount of work to do each day, and you can complete that work AND spend an hour on a social networking site, then why not? Why don't offices let their employees use social networking sites if their work is done?

The two things that motivate human beings is the need to grow, and the need to avoid pain.

Some folks have needed tasks to complete at "work", but view it as painful, and they also see the social networks as meeting growth needs. This is like untimed bathroom breaks, and coffee breaks, and lunches. For some people, they go on and on.