Facebook Exchange is one of the biggest channels for large, direct response marketers since paid search. It allows marketers to re-target potential customers when they return to Facebook.
Specifically, Facebook Exchange is a way to purchase Facebook Ads through real-time bidding. It allows for advertisers and agencies to use cookie-based targeting through Demand-Side Platforms (DSPs) to reach their audiences through Facebook and timely and relevant messaging. For brands and agencies, the result is a powerful tool for driving direct response goals on Facebook.
Take this example, say that you are an online retailer and a customer has put an item into their cart, but leaves the site right before the transaction. Facebook Exchange allows you display ads on Facebook retargeting these customers, providing them with links to return to your site and go through with the transaction.