Very interesting article on what we know about the effectiveness of online advertising. The answer is that we don't know much, except that it's probably not nearly as effective as people assume. https://thecorrespondent.com/100/the-new-dot-com-bubble-is-here-its-called-online-advertising/13228924500-22d5fd24 …
The nut graf is there even if the author doesn't recognize it - the modern corp is totally consumed by principal-agent problems and the online ad budget is just the tip of the iceberg that you can use to prove the existence of the phenomenon