There's some nuance here. The ad network whose script/iframe is directly injected onto the top frame can know fairly easily, since they know which accounts sho

There's some nuance here. The ad network whose script/iframe is directly injected onto the top frame can know fairly easily, since they know which accounts should be linked to which domains, however when these inevitably get resold, these subsequent ad networks have a much tougher time. What ends up happening is that the first thing that gets injected is someone totally shady ad network that doesn't care, who will then resell it to someone slightly less shady, who will bundle it with a pile of other traffic, etc, etc, until it's been laundered enough times that it's hard to separate this out.