It's really down to the Head of Department or senior manager whether these sites should be accessed by staff.
There are many reasons to allow them, for instance 'LinkedIn' is useful for business contacts, and Facebook is good for recruiting (whether the latter is morally right or wrong is irrelevant, the fact is, it's there and employers will use it).
The role of IT should be to work with senior management to decide whether staff should be allowed access, and then advise management if they are aware of misuse. On that basis, the IT Department should have the necessary equipment to control and monitor access, for instance, a 'Network Box' device would be appropriate. You may find this White Paper useful...
www.network-box.co.uk/sites/default/file..._social_networks.pdf
Colin