Amazon, Snapchat, Twitter, Instagram, LinkedIn, Pinterest and Foursquare were all put on notice by an advertiser group demanding they open up their walled gardens.
The digital advertising platforms have yet to adopt independent audits of ad campaigns, according to a new report by industry group ANA.
Google and Facebook recently announced plans to allow audits by the Media Rating Council, an advertising watchdog group. The rating council can verify ad campaign data such as audience size and other metrics, and does not rely on a platform's own analysis.
"Embrace the transparency," said Bill Duggan, exec VP, ANA.
The ANA surveyed 113 of its members from the ad industry, and 89% viewed independent audits as positive. The rest were neutral or likely had some privacy concerns about tracking consumers, Mr. Duggan said. Though he said that's not an issue for audits.
Last month, Google and Facebook announced they would commit to verifying ad campaign data with the rating council. The moves followed a call by Marc Pritchard, Procter & Gamble's global marketing officer, to clean up the digital media supply chain.
Advertisers have less confidence in online ads since the industry suffered years of fraudulent traffic and uncertain viewability standards around how the ads appear on screens.
Last year, Facebook made a commitment to providing better measurements after it found flaws with its reporting on unpaid video views and other data points. The disclosure kicked off a wave of advertiser demands for more accountability throughout the industry.
"Amazon, Foursquare, Instagram, LinkedIn, Pinterest, Snapchat and Twitter: The ANA calls on these organizations to embrace transparency and break down their walled gardens," the ANA said in its survey.
Not all the companies could be reached for comment, but some are in talks to formally open to MRC scrutiny. Twitter, for instance, is in active conversations with MRC, according to people with knowledge of the talks.
Meanwhile, Snapchat is open to third-party ad measurement from Moat and Integral Ad Science, and other partners. Audits would help verify what those firms report.
As for Instagram, it is owned by Facebook but is not under the same auditing regime, according to ANA.
"One frustration of marketers is they are unable to optimize if walled gardens don't open up and share data," Mr. Duggan said.
Pinterest said it is in talks with MRC and auditors but is focusing on partners like Moat, which recently joined its marketing partners program.
"Our advertisers and agency partners are supportive of this approach and prioritization," a Pinterest spokesman said in an email. "We do believe we will conduct audits in the future and are committed to third-party measurement."