Is to set meaningful goals for the staff who report to you and mange them against those goals and objectives. This has the added benefit of managing their smoke breaks, chats with coworkers, long-lunches, newspaper reading, etc. etc.
I'm not advocating giving your staff so much to do they are buckling under with it, but if you have a well managed work force they won't have time to be slacking off.
Any downtime they have when meeting expected targets is just part of the natural eb and flow of working patterns. None of us get through the day without chatting to a friendly coworker, checking the scores, whatever it is we are interested in.
If you have a company where a significant % of your staff are slacking off routinely then you have a badly managed workforce. The problem is not internet access, facebook, monitoring tools, etc. it is the management of the business.
Even if you ban it right off your network people can use smartphones, iPads, whatever to access Facebook, Twitter, LinkedIn, WebMail, IM, etc. without you ever tracking that. other than lead lining the walls of your office you can't stop it either.
OK, there is a different take on this if you are talking about the security of your network and risks through malware, data leakage, all that which may be percieved via social networking et al. That is where the IT part of this comes it. Managing workforce productivity is the job of business unit managers.
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