- AcmeShop is paying Facebook money to run a targeted marketing campaign on Facebook
- to do this, in practice AcmeShop is forced to share visit data with Facebook (this bit could be done differently, but for some reason that's how Facebook does it)
- a customer goes to AcmeShop, doesn't make an immediate purchase, leaves
- AcmeShop is paying Facebook to run a remarketing campaign for that customer, showing them the same product leading to AcmeShop
- at the same time, Facebook is using information that they obtained in the process of providing paid services to AcmeShop to show goods on their marketplace platform to the customer. For Facebook, it's not "remarketing", it's just "marketing" at this point. Not only that, but they control which ads they are showing, and naturally with Marketplace-only placements Facebook has an advantage over AcmeShop (again, using information that they effectively forced AcmeShop to provide)
Does this reframing make the problem of Facebook being both an ad seller, an ad manager, and offering the same service as their customers any clearer?