The Be-All and End-All Metric for Evaluating Online Ad Performance

Impressions
If you’re optimizing a campaign around impressions, the goal quickly shifts from impacting consumers to delivering the highest volume of ads possible within your budget. Theoretically, more ads means more brand exposure and, ultimately, more conversions. But in order to maximize the amount of budget that can be spent on increasing impressions, advertising partners often dump ads into cheap inventory — e.g., below the fold or layered beneath other ads — where they're unlikely to be seen by consumers. With limited consumer visibility, it’s unlikely that an ad campaign run on impressions will make much of an impact on consumers.