Thousands of advertisers want to show their ads to different Facebook users (target audience)
#2 Determining Bids
Facebook holds auctions that take into account each of these advertisers’ bids for an opportunity to serve an ad impression. Facebook also considers how interesting and relevant the target audience will find each of these ads, and how likely they would take an action such as clicking or making a purchase after seeing the ad.
#3 Choosing The Winner
Based on all of these considerations, Facebook determines which advertiser will win this auction.
#4 The Audience Starts seeing The Ads
Finally, Facebook starts showing the ads on the newsfeed of the targeted audience.