One kind of fraud that I suspect exist but I hear less talk about here is fraud against advertisers;

Last time I clicked and bought something from an ad was 5 years ago. It costed me about USD 30. Either I'm totally extraordinary (not likely) or this is true for huge parts of the rest of ad viewers as well. I also suspect that there is a connection between disposable income and resistance against ads.

More specifically I suspect ad networks fleece dumb and inexperienced ad buyers buy selling ads they know cannot work.