Correct, we don't know who shared until someone visits the link. But, we can build a custom audience after the fact because the Facebook retargeting pixel lets

Correct, we don't know who shared until someone visits the link. But, we can build a custom audience after the fact because the Facebook retargeting pixel lets you pass an arbitrary ID of your choosing with each pixel load (the variable name is "extern_id" [1]). So when a click comes in on a given URL that we know had to have been shared, we know what extern_id we gave to Facebook for the original user that shared the link on the pixel fire back when they first visited the site. We can then build a custom audience using a list of those extern_id's for only people that have shared, after the fact. https://developers.facebook.com/docs/marketing-api/audiences-api/ https://developers.facebook.com/docs/marketing-api/audiences... - see "External Identifiers"