Let me know which type of industry you are dealing with, number of employees in your company first, but on average you may try below points for improvement:
1-Issue a code of conduct/policy to not avoid using personal id's in office hours,
2-It may be allowed in lunch breaks (for few minutes)
3-Issue a notice and take signatures of over all employees ( social site surfers/non surfers commonly) to not use social sites in working hours
4- Keep monitoring them some how technicaly /physicaly/via camera's
5-Put counselling sessions in starting time covering agenda's as " how to give our 100 % in office ,how to focus on our work, how to improve i am the best employee of company.
6-Various management games/plays can be applied to overcome this issue, slowly but sooner or later these things will work and changes can be seen in office.
7- If top management agrees you may also issue a notice displaying if regularly personal id's /social sites found operated in office hours, impact of such activities will display in salary figures.
As this is a big problem these days in mostly maximum companies, if you will use strict using net on computers it may be easily used on personal phones, as a HR you have to create an very strict and professional environment via counselling sessions,training's, awareness programmes and so on.
Sure it will take soem time but slowly you can over come this problem if caught ,knocked, analysed person/situations and treated dynamically through various management techniques .
ISO Consultant & HR
+91 - 96707- 66 888
27th September 2014 From India, Lucknow