Based on the Chapter $12$ lecture and readings, Facebook's ad network customers are billed based on $q,1.$

Number of times Facebook users click on displayed ad links $($CPC$)$

Number of user engagements the ads receive $($e$.$g$.$ likes$)$

Number of clicks the ad receives $/$ total times ad is shown $($click$-$through rate$)$

Number of times their ads are shown on Facebook $($CPM$)$

$\%$ of sales generated from clicks of displayed ads $($CPA$)$