IDFA

The IDFA is the unique advertising ID Apple assigns to each iPhone, and since ATT you only see it when the user opts in.

IDFA stands for Identifier for Advertisers. It is a string tied to the device that lets an ad platform say the person who saw this ad is the same person who installed the app. Before 2021 every app read it by default. Now it comes back blank unless the user taps Allow on the ATT prompt.

Picture 100 iPhone users who click your ad and install. For the 30 who opted in, Meta receives the IDFA and reports those installs against the exact ad set. For the 70 who declined, the IDFA arrives as all zeros, so those installs fall into your modeled or SKAdNetwork buckets instead. Pull your attribution report and split installs by has IDFA versus no IDFA; the first group is what you can optimize on directly.

A high opt-in rate does not make the IDFA permanent. A user can reset it or revoke permission in Settings at any time, which breaks the match on anyone you were already following.

The IDFA still exists; your access to it is what got expensive.