YES and NO
YES if it is a site like this one where we exchange work related information.
NO if it is a site for getting a date or for telling the world what you had for breakfast.
Good question!! And I think the answer to this can be a very personal one as it totally depends on how stringent your network security policy needs to be. Also what sort of data your staff are working with and indeed what sort of staff you have!!
In some companies access to the outside world simply cannot be permitted due to the sensitivity of data and staff aren’t even allowed mobile phones through fear they could take photos of their screens.
A good IT dept should have some monitoring software in place though and have the ability to determine if staff are spending more time networking socially as a preference to working. Assuming you can police access to such sites and maintain that happy balance with you staff then in my opinion its fine.
Horses for courses though, for sure!!