Google FLoC (Federated Learning of Cohorts)

Google FLoC, or Federated Learning of Cohorts, was a controversial advertising technology developed by Google to replace third-party cookies. The idea behind FLoC was to group users into “cohorts” based on similar browsing behavior, enabling interest-based ad targeting without tracking individuals across the web.

However, FLoC has since been scrapped due to widespread criticism. It was replaced by the Topics API, part of Google’s broader Privacy Sandbox initiative.

What is Google FLoC?

Google FLoC was designed to preserve interest-based advertising without exposing individual user data. Instead of assigning each user a unique identifier, FLoC placed users into groups—called cohorts—based on their web activity.

Each cohort represented a group of users with similar browsing patterns, such as people who frequently visit fitness or cooking sites. Advertisers would then target cohorts, not individuals.

It was Google’s answer to increasing privacy concerns and cookie deprecation, offering an alternative tracking model that ran directly in the browser.

How Did FLoC Work?

Rather than sending browsing data to external servers, FLoC processed user history locally in the browser. The algorithm used machine learning to assign users to a cohort with similar interests. Advertisers only saw the cohort ID, not the full browsing history.

Key Elements:

• Federated Learning: This refers to training algorithms across many decentralized devices without transferring raw data to a central server.
• Cohorts: Groups of thousands of users with similar behavior.
• Cohort ID: A numeric label representing a user’s interest group, shared with advertisers.
• No Cross-Site Tracking: FLoC aimed to reduce fingerprinting risks and improve privacy.

Why Was FLoC Controversial?

Although FLoC tried to improve user privacy, it sparked backlash from multiple corners of the internet.

Major Concerns:

1. Fingerprinting Risks: Even though FLoC didn’t track individuals, combining a cohort ID with other browser attributes made fingerprinting easier.
2. Lack of Consent: Many users were enrolled in FLoC trials without clear consent or understanding.
3. Discrimination & Profiling: Critics worried that grouping users based on behavior could lead to discriminatory targeting—similar to how credit scores or health profiles are misused.
4. Pushback from Browsers and Platforms: Browsers like Firefox, Brave, Vivaldi, and Safari refused to adopt FLoC. WordPress even suggested blocking it by default.

FLoC vs Topics API

FLoC was ultimately abandoned in January 2022, replaced by the Topics API, which became the primary solution under the Privacy Sandbox umbrella.