Are your thoughts that Facebook et al would - instead of charging for an ad 'impression' - charge ecommerce sites a sales commission for the referral? (Disclaimer; I think/imagine they do this already to some extent, but it's not the main revenue driver.)

If this were the case, then it would absolutely be in Facebook's interest to present ads based on a persons likelihood of purchase (perhaps using a deep neural recommendations network (akin to YouTube's) to power it all).