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Several major brands are Watch RK Prime 29 Onlinescrubbing their ads from far-right news outlet Breitbart following pressure from social media users.

A makeshift campaign at the center of the boycott effort called Sleeping Giants claims that more than 20 advertisers have either publicly or privately agreed to remove ads from the site, including Kellogg, U.S. Bank and Warby Parker.

Their decisions come amid renewed backlash against the site prompted by Donald Trump's recent appointment of Breitbart chairman Stephen Bannon as his chief strategist. Critics say the agitative outlet frequently traffics in racist, misogynist and homophobic content.

Kellogg, the most notable advertiser to join the fray, said in a statement this week that Breitbart isn't "aligned with [its] values as a company."

"We learned from consumers that ads were placed on Breitbart.com and decided to discontinue advertising there," a spokesperson said.

That pronouncement caused particular consternation from Breitbart, which breathlessly denounced the cereal maker "un-American" and called for a boycott of its products in a post on Wednesday. As of that afternoon, the hashtag #DumpKelloggs had climbed its way into the top Twitter trends in the United States.

Mashablecontacted many of the other brands named by the campaign to confirm the removal and will update this post once they respond.

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The exodus follows last week's news that AppNexus, one of the biggest advertising platforms on the web, has banned Breitbart from its ad service for alleged hate speech.

The man behind the campaign, who declined to be named for fear of harassment, said he started the effort around two weeks ago with the intent of boycotting advertisers that appeared on Breitbart.

But he quickly found that because of the way third-party networks and automated ad exchanges work, many of these companies had no idea their ads were on the site. The campaign then became more about notifying the marketers of the placements.

He says the impetus went beyond left and right politics; it's about condemning content that is blatantly hateful or has the potential to incite violence or harassment.

Shannon Coulter, who maintains a widely cited list of companies that do business with Trump brands called #GrabYourWallet, also helped promote the effort among her follower base.

A few advertisers have announced their participation on Twitter.

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Breitbart News Editor-in-Chief Alexander Marlow called Kellogg's decision an "act of discrimination and intense prejudice" in a vitriolic statement posted to the site.

"They insult our incredibly diverse staff and spit in the face of our 45,000,000 highly engaged, highly perceptive, highly loyal readers, many of whom are Kellogg’s customers," Marlow said.

However, some have noted the campaign's resemblance to a similar effort Breitbart co-signed against now-defunct Gawker after the site insulted the so-called #GamerGate movement.

Caught unaware

The lack of awareness among brands speaks to a broader problem with programatic ads -- those placed automatically by software -- in general. That is, digital advertisers oftentimes don't have much control over the sites where their ads might surface. Even specified blacklists of sites to avoid don't always work, as Digidaynotes.

While big purveyors of this sort of advertising, like Google and AppNexus, maintain quality standards that exclude things like pornography, gambling, piracy and hate speech, controversial sites or content that clashes with brand values are another matter.

Google's AdSense ad network has declined to comment when asked if it plans to bar Breitbart from the service.

This blindspot may become more prominent as media attention turns to the host of fake new, hate speech and propaganda sites that enjoy floods of traffic from online platforms.