Facebook kills productivity

I've already fallen into the Facebook productivity sink-hole, and so has Jeanine.

I've been thinking about the impact that Facebook would have on the work ethic of all the people making use of it, and my conclusions weren't very good.

Duncan McLeod comments on a statement that he received from Gartner. The statement comes to the same conclusion that many Facebook users have already come to.

Is this really that much of a problem? Surely Social Networking Sites can be used for something productive?

I know, from first hand experience, that LinkedIn is being used heavily by recruiters from technology companies.
Thats useful, right?

Should companies be blocking Social Networking Sites during office hours, or should they let their employees decide at their own discretion?
If someone is going to neglect their work, then surely that should be addressed, instead of skirting around the issue by removing the distraction?