https://www.facebook.com/business/help/1090330204367211
An Engagement Custom Audience is a Custom Audience made up of people who have engaged with your content on Facebook. “Engagement” refers to actions like spending time viewing your videos or opening your lead form or Canvas. Using Engagement Custom Audiences, you can retarget ads to people who have taken these actions. You can also use it as a source for a Lookalike Audience, which will let you find people who are similar to those who’ve engaged with content on Facebook.
