The bottom line here is - how many people click on your ad and how many of those clicks then turn into an actual purchase or request for an offer. There are different indicators of success. The most common ones are: CTR: how many people saw the ad and how many people actually clicked on it, conversions: the number of people who fulfilled your set goal (purchase, inquiry, subscription to the newsletter...), CPC: how much does it cost to get one conversion, ROI: measures the profit or loss, relative to the investment.