The top three reasons respondents advertise on Amazon are: strong performance and return on investment (ROI), size of the audience, and their target audience is active on Amazon. Of those serving ads on Amazon, an average of 30 percent of online sales come through that channel. Marketers also say it outperforms both Google and Facebook/Instagram on key performance indicators, including: "It's uncertain if the ROAS and cost benefits retail marketers are seeing with Amazon over the Duopoly will maintain as Amazon matures as an ad network," Kelly added. "There's very likely a real first mover advantage that brands are experiencing with Amazon. As we begin to move past the early adopters, and advertising on Amazon starts to get adopted by the masses like that of the Duopoly, the channel could become less effective for retailers as more competition will drive up prices thus lowering ROI." Despite Amazon's proven results, marketers have expressed concerns about the platform.