"How does anyone know for sure that all of that traffic (or at least the meta data related to the traffic) is not prepped for data mining on a per-user basis ?"

You can't, but that's not really the point. The point is that it's not permanent in a form that other users can access. Sure, maybe Snapchat stores every picture I ever sent. That sucks and should stop. But at least Grandma can't go back and look at some 2 year old picture I took with some buddies.