Never post your pictures on the sites where any unknown person can see it without your permission.
Never add any unknown person to your contact just because he/she belonged to your institution.
Never post your address, telephone no., mobile no., email id on such sites where it is easily accessible by unAuthorized Persons.
Do not reply to Strangers, if they still try to communicate without you, Report to Moderator or Administrator of the Respective Social Site.
If matter is Serious then Report to Cyber Police too. Do not forget to note down the Profile Id and/or any Identity of the Culprit.
Think almost thrice before joining any group on the Social Sites, so that you do not repent later.
Utmost Safety should always be the first priority, whenever on a Social Site.