Full Answer

Before iOS 14.5 (April 2021), Meta tracked iOS users across apps and websites by default via their device's IDFA (Identifier for Advertisers). Post-ATT, users who opt out no longer generate IDFA-based attribution data. Meta reports that this has significantly reduced signal fidelity for iOS-originated campaigns.

The impact is asymmetric: Facebook Pixel (browser-based) still fires for opted-out users when they visit your website. But Meta can't link that website visit back to the Facebook ad click that drove it, because the IDFA connection is gone. The conversion appears unattributed or is modeled statistically. Modeled conversions are estimates — real purchase-to-ad attribution is lost.

CAPI partially recovers this. When your server sends a purchase event with hashed email or phone number, Meta can match that to the user's account via identity matching — even if IDFA tracking is disabled. This is why CAPI setup quality (EMQ, customer information completeness) matters so much for iOS-heavy audiences.

For WooCommerce stores with significant mobile traffic, iOS tracking loss is often the single largest contributor to underreported ROAS.