Add to the list of people frustrated with Facebook Inc. FB (-2.61%) and Google a quiet but hugely influential group—the people who pay the bills.

In the past year and a half, the two firms have had one run-in after another with advertisers . Procter & Gamble Co. PG 1.76% was among many companies that boycotted Google’s YouTube when they discovered ads were running before extremist and racist videos. Marketers pushed Facebook to provide more credible data about how many people actually view ads on their platforms, after the social giant acknowledged mistakes in its numbers.

Now there’s the revelation that data on tens of millions of Facebook users were improperly accessed by Cambridge Analytica, a firm tied to Donald Trump’s 2016 presidential campaign. That has sparked anxiety among some marketers, including over whether their own data on Facebook are safe.

Madison Avenue’s increasing uneasiness with the platforms and its moves to push back aggressively are fundamentally reshaping the relationship. Advertisers’ broad push for changes has played out in behind-the-scenes dust-ups, veiled and overt threats and advertising boycotts, and has extracted some concessions from the tech giants. Among the leaders is P&G, the world’s largest advertiser.

Many companies are actively policing their ad purchases to ensure they avoid objectionable or irrelevant content. Some are cutting budgets. And they are demanding far more transparency from Google and Facebook about the performance of their ad campaigns to make sure they aren’t wasting money.

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