1. CONNECTED EMPLOYEES ARE MORE EFFECTIVE. Nobody can work in complete isolation, whatever they may think. To be effective and productive you rely on help from other people and the best way to be sure of getting that help is to have a strong network of people who can answer your questions or proactively share with you a useful new method, tool or idea. This is maybe intuitive but if your intuition doesn't get it then I recommend you read How Bell Labs Creates Star Performers, the Harvard Business Review article about Robert Kelley and Janet Caplan's 1993 study at Bell Labs that demonstrated this point. Connecting for performance works best with knowledge workers, but everyone else benefits, too.
2. CONNECTED EMPLOYEES BOOST YOUR REPUTATION. People who are connected and use social media can boost the viability and reputation in many different ways, and much more cheaply than paid community managers. Very often they amplify your organizations's messages, sharing your posts, tweets and updates with their friends. Just by being active they also send a clear message to the community that you are a good employer and they can bring visibility to new audiences you perhaps don't target or don't know how to target. Even if an employee does not comment on your business most likely they have written "Works at ..." on their profiles, which can be seen by people looking for a good place to work or interested in your products.
3. CONNECTED EMPLOYEES ARE MORE MOTIVATED. In theory you could get the maximum possible productivity out of people by banning any kind of break or recreation, but in reality this just makes people stay at their desks. Experience has shown that people actually work more when they get breaks to recharge their batteries, talk to colleagues, get away from the work for a moment. That's why even the most repressive organizations allow their employees coffee breaks, once seen a waste of time but now pretty much universally accepted. When employees can take little breaks to keep up to date with their world they return to their work with new energy. There will always be a small minority who abuse this opportunity, just like there have always been people who tool too many coffee breaks, make too many personal photocopies, talk too much with colleagues and so on.
Clearly there are also some downside risks, though these can mostly be avoided by effective training and communication. Simply banning access could eliminate some risks but it also costs you the benefits. In any case you still have to do the training and communication to make sure employees know how to use social media safely because they can always use smartphones in the workplace or use the social media at home.
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Lectures, Workshops, Coaching, Writing