We already have an emergency messaging system called Amber Alert. Just generalize that infrastructure a bit and you're halfway there.
The current system is used in a suboptimal way, at least by authorities in my state.
The overwhelming majority -- over 90% -- of children being abducted are taken by family[0], usually as a part of a custody dispute.
Yeah screw that haha, I'll take the Twitter clone for anything short of actual emergency.