The report was based on an analysis of millions of keyword searches in 2014.
The researchers looked at Google AdWord PPC (pay-per-click) placements, which are primarily text-based units that are served above and to the side of organic search results, and Google Shopping PLAs (product listing ads), which are primarily photo-based units that are displayed in results for product searches.
The rankings were compiled by assigning each advertiser a PLA Visibility score and a PPC Visibility score, which were based on the number of times each domain appeared in the keyword results examined, its rank within those results, the competitiveness of the keywords targeted, and the probability of click-through.
After Amazon, the most visible Google AdWords advertisers are Wow.com, Ask.com, WebCrawler.com, and About.com.
After eBay, the most visible advertisers on Google Shopping are Wal-Mart, Rakuten, Macy's, and Etsy.
About the research: The report was based on an analysis of millions of keyword searches in 2014.