AdGooroo has released a list of the top 20 advertisers (by impression) using Google product listing ads (PLAs).

“Walmart tops the list with 408 million PLA impressions representing 129,381 unique products with 262,588 unique product listing ads,” a spokesperson for AdGooroo tells WebProNews. “Another surprise is Amazon (the biggest paid search advertiser) falls well short of the top 20; Amazon ranked #75 in the analysis and apparently is not embracing the PLA format.”

When the PLA-based Google Shopping launched last year, it was indeed evident that Amazon wasn’t much interested in taking part, even if other e-commerce competitors are among the top advertisers on this list. Amazon, of course, has its own product ads, and is often the starting point for shopping searches to begin with (Disclosure: Amazon is a sponsor).

Here’s the full list:

Overstock’s entry at number five is particularly interesting, given the company’s history with Google. If you’ll recall, the site was hit with a Google penalty on the organic search side of things after encouraging college sites to post links to Overstock pages so students and faculty could get discounts. When the company announced its quarterly earnings in April of 2012, it blamed Google for an “ugly year”.

Google’s product listings were first introduced in 2010, but have been tending steadily upward in popularity. Google’s shopping search transition to the PLA model has no doubt played a significant role in that over the past year or so, which has led to Google competitors following suit.

More on the list from AdGooroo here.