Since 2011, AdBlock Plus, a popular browser plugin that blocks online ads, has kept a “whitelist” of websites that are allowed to serve ads despite the presence of the AdBlock Plus plugin. In an e-mail to Ars, AdBlock Plus Communications Manager Ben Williams wrote that currently, the browser extension has granted a pass to "over 300 sites/entities" out of "over 1,500 applicants" to the company's whitelist. That's up from October 2013, when AdBlock Plus allowed the ads of 78 sites or entities out of 777 applicants.

To be placed on AdBlock Plus’ whitelist, advertisements must be transparent about being ads, must be appropriate to the site they’re being served on, and must not distort or disrupt the page content, among other criteria. AdBlock Plus goes into more detail about the whitelisting process here.

But one important facet of the business plan is that if a company is big enough, AdBlock Plus' parent company, German start-up Eyeo, asks for a fee in addition to adhering to the "Acceptable Ads" criteria before it allows a company to be whitelisted.

That creates an interesting ecosystem between major companies whose bottom line depends on advertising and the increasingly popular AdBlock Plus. (According to PageFair and Adobe, AdBlock Plus has 144 million active users [PDF]).

On Sunday, the Financial Times confirmed paid deals between Google, Microsoft, Amazon, and ad-serving company Taboola with AdBlock Plus. Although all four companies have been publicly listed in the forums for years as having had ads whitelisted, the confirmation of a monetary exchange shows that these major companies see ad blocking as a hit to their bottom lines.

For smaller websites, whitelisting is usually free as long as certain criteria are met as outlined in AdBlock Plus’ “Acceptable Ads Initiative." In 2013, AdBlock Plus wrote that only 10 percent of its whitelisted ads had paid to be whitelisted. According to Williams, that number remains about the same today: less than 10 percent of the sites and entities whitelisted by AdBlock Plus have paid spots on the whitelist.

It’s unclear how much Google, Microsoft, Amazon, or Taboola has paid Eyeo. The Financial Times reports that one digital media company (which asked not to be named) was told that it would cost 30 percent of its advertising revenue to be whitelisted by Eyeo and AdBlock Plus.

AdBlock Plus users are able to disable whitelisting, but whitelisted ads appear by default. Although some users of AdBlock Plus have found the additions surprising and unwelcome, AdBlock Plus encourages users to report ads that don’t comply with the company’s Acceptable Ads Initiative.

Still, exchanging money with ad-serving companies may make users wary of how far AdBlock Plus will go. In a New York Times article published in 2012 when AdBlock Plus was launching its Acceptable Ads Initiative, Eyeo Managing Director Till Faida said at the time that none of the whitelisted ads had paid to be whitelisted, adding, “we haven’t sold out in any way.”

And although AdBlock Plus administrators post which sites and entities they are whitelisting in the AdBlock public forum, it's often not clear which companies are paying Eyeo for the privilege of being on the whitelist and which are getting a whitelist spot for free.

On the other hand, Google has paid Eyeo to have its ads, as well as its ads delivered through AdSense for Search partner sites, whitelisted for two years without much issue.

In a comment to Ars Technica, Williams wrote, "In general, it's a little surprising that people are so surprised."

"To be as transparent as possible, we've done everything short of taking out a Super Bowl ad to inform people about our Acceptable Ads initiative,” he added, listing blog posts and podcasts that the company has published in the last several years.

Williams continued, "All in all, I'd just like to emphasize that in the absolutist world of adblocking-vs-online-ads, we're proud to offer a compromise that can encourage better ads and leaves users in control over their online experience.”

Ad blocking continues to be a contentious topic. While proponents want to be able to browse the Web distraction-free, content-creating websites (like Ars Technica) derive the bulk of their revenue from serving ads to viewers. That revenue in turn enables them (and us) to produce more compelling content and handle the traffic load from all the people who want to read those articles—including the traffic from those who block ads.

Google and Taboola would not comment to Ars about the report. Ars also contacted Microsoft and Amazon for comment but did not receive a response.