Title: Competing ad auctions
Description: We model competing auctions for online advertising, with attention to the participation costs that limit advertisers' interest in using small ad platforms. When participation costs are large relative to the volume of traffic an ad platform can offer, an advertiser may forego use of an ad platform that the advertiser otherwise finds profitable. Mergers between ad platforms can increase advertiser welfare if the resulting click-through rate and volume of traffic are sufficiently improved relative to the offerings of the ad auctions when separate. When there is an insufficient improvement, such mergers can harm advertisers.