To help advertisers better understand how well their ads are competing in the auction, Facebook is rolling out Delivery Insights in Ads Manager. The new tool surfaces ad sets that are under-delivering and not reaching as many people as they could be. Delivery Insights gives the advertiser information about why the ad set it is under-performing and offers optimization recommendations to make the ad sets more competitive in the auction.
Source: Facebook launches Delivery Insights to help advertisers make better ads