Facebook has said it could lose most of its revenue from ads on apps, because of changes to Apple's operating system.
In the upcoming iOS 14, apps have to explicitly ask users' permission to collect and share data, meaning ads will no longer be able to just "follow" users to apps outside of Facebook.
Users targeted in this way are eight times more likely to buy products, according to Facebook.
But iOS 14 tests suggest "more than a 50% drop in... publisher revenue".
Audience Network, which Facebook describes as a "way for advertisers to extend their campaigns beyond Facebook and into other mobile apps", works by using an identifier code to target advertising on the basis of users' "likes" and preferences.
IPhone users can already opt out of this tracking.
But Apple's plan to require all users to actively opt in before they can be tracked "may render Audience Network so ineffective on iOS 14 that it may not make sense to offer it", Facebook said.
In a blog, Facebook warned advertisers they "should expect their ability to effectively monetise from Audience Network to decrease" and called for industry consultation on the changes, which it said would have "a far-reaching impact on the developer ecosystem".