How can we state in an employment Contract that use of the internet for social networking e.g. using Facebook and Twitter is strictly prohibited.
It is perfectly in order to place such a restriction within the statement of terms and conditions that you will issue to the employee when they commence working with you. However this will not affect your current employees unless you issue them with a contract amendment letter stipulating the new condition.
A better plan would be to incorporate the restriction into your staff handbook if you have one, or even better issue a proper policy on the use of the internet. This would explain much more thoroughly than a single paragraph in the contract and could describe exactly what restrictions you want to place on your staff with regard to these sites, as well as other issues like accessing pornography or abuse of your email system. You can then issue the policy to each member of staff and ask them to sign a form for their file confirming they have read and understood the policy.
There is a template internet & email policy on our website here which you can download for free and adapt for your purposes.
(Article Reviewed March 2018)