Does your company have a formal policy on employee social media usage during the work day (or after)? If not, you’re not alone: a report from employment services firm Manpower shows that only 29% of companies have a formal social media policy in place.

Obviously, far more companies block popular social media sites—another study in October showed that half of all companies block YouTube, Facebook and/or Twitter. But, as Mashable points out, those two stats aren’t mutually exclusive: there’s a difference between a written policy and simply avoiding the issue with blocking sites.

Most companies with a written policy (63%) say that it’s effective. But social media policies can cover much more than just work hours. They can also cover the company’s assets: in a written social media policy, you can establish guidelines for what employees can and can’t mention in social media (trade secrets and insider trading type stuff should be obvious, but isn’t always—and naturally, these rules can go too far, and could become the subject of unlawful termination lawsuits. Woot.).

I’m pro written policies: make it clear that your employees are not to spend work time watching YouTube videos, and enforce it. (Fire them, if that’s the policy.) However, this gets a little trickier when you’re using social media for business purposes, as more and more of us are—and even more tricky when you got on LinkedIn to send a message to get the contact info for a vendor and get sucked into the Q&A for two hours. (Apparently, most workers spend less than 30 minutes a day on social networking sites—which may be why most employers haven’t felt it necessary to come out with formal policies. Yet.)

What do you think? Should companies have formal social media usage policies for employees?