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?

Answer 1

As my father told me many years ago:

"What do you mean you had nothing to do? There's always something to do at work!"

If a person is paid for 8 hours of work, then morally, legally, and ethically, he owes those 8 hours to his employers. Period.

Answer 2

Another thought -- if there isn't enough work to do -- then the employer may decide to cut staff so that each person has plenty of work each day. Goofing off at work is likely to cost people their jobs.