You can do that with first-party cookies though. Allow some facebook supplied javascript to set a first party cookie when the user hits website.com/ad-34, and

You can do that with first-party cookies though. Allow some facebook supplied javascript to set a first party cookie when the user hits website.com/ad-34, and then reference that 1st party cookie the next day when the user returns. Phone home when they buy something. Facebook can now correlate (1) the ping they received to begin with, (2) the user session cookie they initiated on the first visit, and (3) the user session they observed when a purchase was made. I'm sure things get easier with 3rd party tracking, but fundamentally you can do it without cross-site tracking.