In 2019, amid a flock of billionaires gathered at the annual Sun Valley retreat in Idaho, Apple's Tim Cook and Facebook's Mark Zuckerberg had an ill-fated meeting, The New York Times reported on Monday.

Zuckerberg asked for Cook's advice on dealing with user-privacy issues in the fallout of the Cambridge Analytica scandal, where data from over 50 million Facebook accounts was harvested, the report said, and Cook's response "stunned" the young Facebook CEO.

The report said Cook instructed Zuckerberg to delete all the user data his company collects from outside of Facebook, Instagram, and WhatsApp.

It was tantamount to Cook telling Zuckerberg that Facebook's core business was "untenable," The Times reported.

Facebook tracks its users all over the web, even when they're not using a Facebook service. That data is critical to Facebook's advertising sales, which is core to how the social-media giant makes money.

Facebook and Apple have sparred publicly for years over privacy issues, going back to at least 2014 when Cook called out the business models of companies such as Google and Facebook in an interview with Charlie Rose. "I think everyone has to ask, how do companies make their money? Follow the money," he said. "And if they're making money mainly by collecting gobs of personal data, I think you have a right to be worried. And you should really understand what's happening to that data."

Most recently, Apple appeared to take a direct shot at Facebook with its iOS 14.5 update, coming this week, which is designed to allow iPhone users worldwide to opt out of tracking. In short, the new update enables iPhone users to stop Facebook from tracking them outside of Facebook's own apps: The same suggestion Cook is said to have given Zuckerberg back in 2019.

Neither Apple nor Facebook responded to a request for comment as of publishing.