“If Russian malware is chatting through Instagram comments, what sort of digital dead drops might be happening on Facebook or Twitter?”
I saw this article recently and it explains very well why authorities are having such a hard time stopping terrorists and why they need big social media companies to help. Although I suppose they could just friend everyone on the planet.
Read about How Russians hid a message in plain site, how they could again and why it is so darn easy to get a message out to many disconnected people.
Read this article from Popular Mechanics
Hiding malware and other messages in plain site.