Moderation is the key
This is one of those things which management needs to assess on a case-by-case basis. Like allowing employees to take 'extra' cigarette breaks, personal phone calls or go a few minutes extra at lunch, anything has the potential to affect the bottom line but employee morale has a huge value as well. Consider it as an advertising expense. We allow it here but there's a very clear 'social media' policy in place and, considering the alternatives, no employee in his or her right mind would argue that the policy is 'unfair'. Like previous writers have said, employees ARE being paid to work, and it's fine to monitor and adjust social media usage as necessary.
