Yes, you can add a clause in the appointment letter on the norms to be followed on social media.
As such companies add a clause in the appointment letter about the applicability of the provisions of all the policies in vogue, the changes made to the policies in the vogue and the policies that may be designed in future. However, this is a too general statement. Over and above this general statement, if you wish to add separate clauses on norms to be followed on social media, non-disclosure of information etc. it will be perfectly normal.
From India, Bangalore
Of late this clause on social media is becoming more crucial. Especially to mention BPO's, Customs bonded warehouse wherein the data is kept and to be imported should be handled cautiously by the employees and any share of such data in social media is to be strictly prohibited. Nowadays International Clients who are outsourcing their projects to Indian Companies add a separate clause on data security itself and any share of such data in social media will be construed as violation of the Agreement entered between Employer and the Client.
From India, Madras