We ran a story a few months ago about a different survey that said more than half of companies block Twitter, Facebook or MySpace. That piece might seem like it conflicts with these numbers, but there's a distinction between a CIO opting to block access to sites and a formal social media policy.
Keeping productivity up is a reason either way; 63% of companies who have a social media policy say it's effective. But a formal policy can also be put in place to make sure an employee doesn't say anything publicly that could damage the company's reputation. The report says that 4% of companies worldwide claim they have had their reputations tarnished by employees' social media activity.
Supposedly most workers spend less than a half an hour a day on social networking sites (Over here at Mashable, we spend much longer than that). If you're a business owner or a manager, take a look at the articles we've written to help you decide if a policy is right for your company and figure out how you should put one together if it is.