Isn't e) how a bunch of sting operations already work? And doesn't really require much cooperation from Facebook - there's nothing stopping people from recording, and given enough evidence (e.g. that the stranger knew (or thought they knew) the user was underage, and continued their sexual advances), this would turn into a police warrant to Facebook to deanonymize the perp. As far as I know Facebook already complies with such requests (and makes it extremely hard to use their products anonymously), so... what more do we want them to do? Isn't the space they're providing much safer than being out in public, for a child?