How iOS 14 Changed Facebook Ads Measurement Forever

What Apple's ATT Framework Did

In April 2021 (iOS 14.5), Apple introduced the App Tracking Transparency (ATT) framework — a requirement that apps must ask users for explicit permission before tracking their activity across apps and websites. The prompt: "Allow [App] to track your activity across other companies' apps and websites?" Users could tap "Ask App Not to Track" to opt out.

The result: approximately 85% of iOS users opted out of tracking when given the choice. This meant Meta's Facebook app could no longer track the post-click behaviour of these users on advertisers' websites — breaking the foundation of how Meta attributed advertising conversions.

What Meta Lost Access To

For the 85% of iOS users who opted out of tracking, Meta can no longer:

• Track users across websites using the Facebook Pixel (the _fbp cookie is blocked or deleted)
• See which users who clicked an ad later made a purchase
• Build accurate conversion-based lookalike audiences from iOS users
• Run effective retargeting ads to iOS users who visited specific pages

Aggregated Event Measurement (AEM)

Meta's response was Aggregated Event Measurement — a new framework for measuring conversions from iOS users with privacy restrictions. Key constraints:

• Advertisers must verify their domain with Meta
• Only 8 conversion events per domain can be used for ad optimisation on iOS (down from unlimited)
• These 8 events must be prioritised in Events Manager (higher priority events take precedence)
• Conversion data for iOS is delayed up to 3 days (aggregated and modelled by Apple)
• Conversion attribution windows changed: 7-day click, 1-day view (previously 28-day click, 28-day view)

What This Means for Your Campaigns

• Reported conversions fell: most advertisers saw 20-40% drops in Meta-reported conversions after iOS 14 — this was not a real drop in sales, just fewer conversions being attributed
• Cost per conversion rose: with fewer conversion signals, Meta's algorithm had less data to optimise on, leading to less efficient delivery
• Retargeting became harder: smaller iOS retargeting audiences due to loss of pixel data
• Attribution modelling introduced: Meta now uses modelled (estimated) data to fill gaps in iOS attribution — reported numbers are partially real, partially modelled

How Server-Side Tracking Helps

The Conversions API partially addresses the iOS 14 impact:

• CAPI sends events server-to-server, not from the browser — ATT opt-outs do not block server-side events
• CAPI can use hashed email and phone to match iOS users who are logged into Meta's apps — a different signal than browser-side cookies
• More conversion signals → Meta's algorithm performs better even within AEM constraints

CAPI does not fully restore pre-iOS 14 measurement — AEM's 8-event limit and modelled attribution remain. But it recovers a meaningful portion of the lost signal.

Summary

iOS 14 fundamentally changed Meta advertising by removing browser-based tracking for the majority of iOS users. AEM constraints (8 events, shorter windows, modelled data) changed how campaigns are measured and optimised. Server-side CAPI implementation partially recovers lost conversion signals through hashed PII matching rather than browser cookies. This remains the best available mitigation — perfect restoration of pre-iOS 14 measurement is not possible within Apple's framework.

See our Facebook CAPI Setup service for implementation.

Need help with post-iOS 14 Meta measurement? Contact Adslytics.