Mobile app advertising on iOS has long relied on Apple's IDFA - Identifier for Advertisers - a random, device-unique, resettable identifier that was available to all apps.
An app like Candy Crush would tell ad networks like FB that "IDFA 123456 just installed the app" or "IDFA 654321 just made a $5 in-app purchase." FB could then recognize that IDFA as a user who installed the FB app, and they could use that data to target, optimize via ML, and measure the audience and results of Candy Crush's marketing campaign.
Apple's recent iOS update made IDFA an app-level opt-in, so in order for this existing advertising mechanism to work, you needed to opt-in to tracking on FB (so they could associate your IDFA to a FB user) AND to opt-in to tracking on the other apps (so they could send ad networks data with your IDFA). This double opt-in obviously has a super low rate.
FB wasn't impacted nearly as much as other advertising businesses because they already have identifiers for their users (email, phone number, etc), have a ton of first-party interest/behavioral data, and they mostly run ads on their own properties. Apps who used third-party ad networks to sell and place ads in their apps had their business completely destroyed.
I think this is all fair in terms of user choice and to give Apple control of their own platform, but it opens Apple up for a lot of regulatory scrutiny given that they were able to flip a switch that annihilated a $50b mobile advertising industry, while making their own equivalent data collection opt-out (vs their competitors' opt-in). Even then, Apple frames their data collection option as "Personalize your experience" while forcing other apps to use the phrase "Allow this app to track you." Apple's own ads product has tripled its market share since this launched. [0]
[0] https://www.ft.com/content/074b881f-a931-4986-888e-2ac53e286...