You are indeed right that this is very hard to measure. It is somewhat easier on social media as advertisers often get detailed information about who got shown which ad at what point and how long it stayed on screen, if they moved their mouse over it, etc. Obviously the easiest indicator for an ads effectiveness is to measure how many people clicked the ad and then how many stayed on your webpage navigating around and then how many actually spent money and how much money they spent. But this does not give very accurate results as a lot of people who see an ad will ignore it initially but then talk about the product more frequently after having seen the ad which then might result in a sale through another channel later on. So you need to try to correlate the different ad campaigns with things like mentions, visits to your webpage, and eventually sales. And this can be very hard to do right.