Optimized CPM, Facebook’s optimization bid type, enables you to choose an objective (or goals) for your advertising. To meet these objectives, for example sending users to your website or promoting a product catalog, Facebook will optimize your ads delivery. With optimized CPM, Facebook will bid on your behalf, distribute your budget, and optimize it to meet your objectives.
When Facebook bids on your behalf, it will remain constrained by the campaign budget you define and the values you specify. The dynamic bids allow the system to capture the highest-value impressions for your goals, and you should expect the total ROI on the campaign to exceed that of either a CPC or a traditional CPM campaign.
Optimized CPM is a bid type that shows your ad to people who are more likely to take the action you want. With this bid type, you pay for impressions (CPM) or the amount of times your ad is shown to a user. For example, if your advertising objective is get more page likes, an optimized CPM bid will be shown to people who are more likely to like your page. Your bid will automatically adjust to help your ads reach your target audience without spending more than your budget.