For the past year and a half, an anonymous activist group called Sleeping Giants has rattled the ad industry by using its popular social media accounts to share screenshots of ads appearing on sites such as Breitbart.com.

There’s no way to definitely quantify the amount of discomfort Sleeping Giants has provoked in CMOs, but its tactics have had the effect of getting thousands of advertisers to pull their ads off of publishers’ websites.

The controversy surrounding Sleeping Giants resurfaced last week when The Daily Caller named the group’s founder, who subsequently gave his first public interview to The New York Times after getting outed. Sleeping Giants did not respond to messages for this story.

Sleeping Giants became popular after the 2016 presidential election. The main targets of Sleeping Giants’ crusade have been conservative publishers such as Breitbart. The group has also blasted advertisers for running ads on some Fox News programs.

When Sleeping Giants first started tweeting its screenshots of ads, many advertisers acted surprised to see their ads run next to controversial articles. They blamed the complicated nature of programmatic ad buying for placing ads in places where they didn’t want them.

The momentum from Sleeping Giants’ protests led many ad buyers to request that their demand-side platforms (DSPs) blacklist Breitbart. Within just a few months of the presidential election, major DSPs like AppNexus, The Trade Desk and Rocket Fuel went from purchasing about 5% of Breitbart’s inventory per month to purchasing a fraction of a single percent, according to ad tracking company Pathmatics. Nearly all of Breitbart’s programmatic inventory is now sold through Google’s ad tech platforms.

Some ads still slipped through after blacklists were implemented, but Sleeping Giants’ activism has had an effect nonetheless. According to Pathmatics, the number of unique advertisers to appear on Breitbart declined 46% from 3,604 advertisers in Q4 2016 to 1,967 advertisers during Q2 2018.

Regardless of what people think of Sleeping Giants’ tactics, the conversation they’ve sparked around the perils of programmatic media buying isn’t going away. In a July survey of 129 ad agency professionals worldwide conducted by Iponweb and ExchangeWire, almost 80% of respondents said that the lack of transparency around media buys is a major challenge of programmatic buying.