https://arstechnica.com/tech-policy/2021/07/facebook-advertisers-are-panicking-after-ios-cuts-off-key-tracking-data/

Facebook’s ability to track users and show them certain ads appears to be tanking thanks to Apple’s “ask not to track” feature, according to some advertisers.

Apple rolled out the privacy prompt in late April with iOS 14.5. Since then, nearly half of all iOS devices worldwide have at least version 14.5 installed, according to Statcounter, and a vast majority of these devices' users have chosen to deny Facebook and other apps the ability to track them. Nearly three months after the feature's launch, just 17 percent of users worldwide have opted in, according to analytics company Flurry.