Custom Audience vs Lookalike Audience: What's the Difference?

A Custom Audience is built from data you already own - a customer list you upload, visitors tracked through the Meta Pixel or Conversions API, people who've used your app, or people who've engaged with your Page or Instagram profile. It targets people who already have a relationship with the business, which makes it the standard tool for retargeting, retention campaigns, and audience exclusions.

A Lookalike Audience is built by Meta, not by you - it analyses a source Custom Audience (your best customers, for example) and finds new people on Facebook and Instagram who share similar characteristics and behaviours. Lookalikes are sized as a percentage of the target country's population, from 1% (the closest match to the source, smallest reach) up to 10% (a looser match, larger reach).

The practical split is retargeting versus prospecting: Custom Audiences reach people who already know you, Lookalikes reach new people who resemble your best existing ones. A Lookalike is only as reliable as its source - Meta recommends a source list of at least 100 people to build one at all, and results are noticeably stronger with 1,000 or more, ideally weighted toward actual customers or high-value converters rather than just anyone who visited a page once.