Tbh I don't know enough about Snapchat to comment, but I do know it played a big role in online bullying. 

Whatsapp...I would say no, not what I mean by social media.

If I understood you correctly, it sounds like the intervention you're talking about is intervention as a parent/individual, not as a platform. Supervising, reading, speaking up when necessary, calling other parents, removing privileges etc 

 Intervention must be at a community level. Centralized or not, platforms can do very little.

I consider WhatsApp a Social Media. For me, it meets the characteristics to be classified as such: participatory, in real time, etc.