Are you with/Against forbid Social Media Sites from workplace?

Social medias should be accessible from workplace. We learn many useful things as well as use a quick break from work stress. Not to mention how important it is to be able to connect with your friends and family from time to time.

I do not think that social media can be helpful for the employees except e-commerce or customer relationship business. It destroys the valuable time for the employees. So I strongly against social media in the  work place.