What is a Custom Audience?

Meta’s Marketing API guide says a Custom Audience can be built from customer information, including email addresses, phone numbers, names, dates of birth, gender, locations, app user IDs, page-scoped user IDs, and Apple and Android advertising IDs. You are the owner of that data and responsible for managing it, including information from your CRM.

To protect privacy, you must share the data in a hashed format. Meta compares it with its own hashed data to see if a person on Facebook should be added to the audience. There is no cap on records in an audience, but the guide says you can add only up to 10,000 at a time, and that changes usually take up to 24 hours.

There is another route that needs no upload. If you share conversion events with the Conversions API, the guide says you can create a website Custom Audience from them.

• From 2 September 2025, Meta said it would flag custom or lookalike audiences that suggest specific health conditions or financial status, such as “diabetes” or “credit score”. Flagged audiences cannot be used in new campaigns.
• Customer file audiences not used in any active ad set for over two years are flagged for deletion on a rolling basis, and you must give instructions on whether to keep them.
• When you create a customer file audience through the API, you say how the data was collected: directly from customers, from partners, or both.

Tip: Name every audience with where the data came from and the date, for example “Buyers Jan 2026, email list”. After a year you will not remember which list a vague “Customers 2” was.

The guide lists page-scoped user IDs among the supported identifiers, so people who messaged your Page can be a source. That is a use of customer data with its own consent and terms. Meta publishes Custom Audience terms of service, and the rules differ by region, so read them before uploading a list.