1 - Referrals: Locating and retaining talent is typically paramount for any business and the best source of quality hires is via employee referrals... so the easiest way to do that is to have employees share your jobs via their social networks and refer their friends to your recruiters.
2 - Performance: if you hire professionals and treat them like school kids you deserve low morale and low performance. If your employees are performing their job and deliverying the results you expect in a professional way then who cares if they hang by the social water cooler more than you think they should.
Besides, most people have FB and Twitter, IM etc. on their mobile phones, are you going to collect them in a bucket when they show up for work?
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