Custom Audience: People Who Already Know You
Say you run a small boutique. People who visited your website or interacted with your Instagram posts have already noticed you once. A Custom Audience lets you introduce these people to Meta's ad system so you can target those who've shown interest — rather than starting cold with total strangers. Common sources include website visitors (tracked via Meta Pixel or Conversions API), existing customer lists (matched by email or phone number), app users, and people who've engaged with your Instagram or Facebook content.
Lookalike Audience: Finding People "Like Them"
Once your Custom Audience exists, Meta analyzes shared traits among those people — age ranges, interests, behavioral patterns — and finds others in your target country who fit a similar profile. You don't search for them one by one; Meta does the matching. You'll typically choose a percentage between 1 and 10: 1% means the closest match and usually performs best. As the percentage grows, the audience gets larger but less similar to your original list.
Meta reports that businesses combining Custom and Lookalike Audiences see roughly double the conversion rate compared to cold interest-based targeting alone.
- Website visitor Custom Audiences — test 30-, 60-, and 180-day windows separately; recent visitors often respond best.
- When uploading a customer list, include both email and phone number to improve the match rate.
- Build Lookalike Audiences from a source of at least 1,000 people; smaller lists don't give Meta enough signal.
- Always exclude your Custom Audience from Lookalike campaigns; otherwise you'll pay to re-target existing customers as if they were new.
- A Lookalike built from buyers typically outperforms one built from all visitors — quality of the source list matters.