“Allot of the issue now around banning facebook is it can be used to promote your company and by banning it you lose this option.”
Exactly, people have enjoyed the visible difference in their increased sales and clients by promoting their business on social media like facebook. If you are also running such a marketing campaign on facebook, then its your functional requirement, how can you block it?
But if you are not using facebook for the sake of business, then you must measure if your employees are making the excessive use of facebook, if it is hurdling in the work productivity, then you must install a stern policy for that.
But if there is a possibility to see any unusual reaction, then you can opt a temperate solution, permit it during the break time only.