Facebook Custom Audiences are well-defined groups of people who already have an established relationship with your business. For example, existing customers and people who have previously visited your website or clicked your ads.
By syncing your subscriber segments with Facebook Custom Audiences, you can:
Show ads to specific segments (e.g. fresh leads, subscribers who finished an automation, etc.)
Exclude certain subscribers from campaign targeting
Create Lookalike Audiences to build targeted campaigns for potential customers that “look like” your existing customers