Ability to add leads in Facebook's custom audience within Workflows.
Use Case 1: When someone signups for trial, he gets added to the custom audience where we show relevant ads to nurture that user for the next 30 days and try to convert him into a customer. When the trial ends after 30 days or the user becomes a customer, he is removed from that custom audience and added into a new cutom audience where "nuturing for customers" ads starts to show.
Use case 2: Contacts with abandoned cart event are added into a custom audience where we show ads to get them back to the cart.