Apple‘s iOS 14 will implement a new privacy rule, where developers will need to ask users’ permission to track them across mobile apps and websites.

However, the new privacy rules won’t be live when iOS 14 launches in autumn, but only sometime in 2021, Apple said in a developer update on Thursday.

Facebook warned the changes will hurt its advertising business. Its Audience Network lets advertisers target users even when they are not using Facebook.

By removing personalization, Facebook said, the new privacy features would cause the Audience Network’s revenues to drop by 50%.

Advertising accounts for the vast majority of Facebook’s business, but its user tracking practices are controversial.