As someone responsible for $xxxxxx monthly spend in online advertising, it clearly works. We measure it pretty much from end to end.
Do you know how much profit you would have earned without the advertising? How do you determine that (if you can say)?
That is, without the ads, some customers would purchase more or more profitable products; some would purchase the same, and some would purchase less. But how would it net out -- and more interesting to me, how do you measure that?