How important is it to have a social media policy?

Q. I am currently conducting the annual review of our employee handbook. I realized that we do not have a written policy on the use of Social Media during or after working hours. Our IT department has eliminated the use of Facebook on company computers. However, other social media websites are accessible, and we are beginning to expand our marketing efforts into areas such as LinkedIn and a company blog. How important is it to include a written Social Media policy in our handbook? – Laurie, Minnesota