What would you do? If you saw a threatening post on social media, vague or persise? If you saw someone like the punk is Oregon dressed daily in army green posting pics of himself with a firearm. Telling his audience "don't go to college spclasses today if you live in the northwest". If this had been reported could have it been stopped? Ask yourself, honestly would you report it?
I would I'd feel like hell if I knew and did nothing.