Each lead either notes if it is from Facebook or Google or Bing has a code.  The code tells us this lead is from a publishing network of 2000 sites.
We do not know on which which site the lead originated.

We subscribe to a publishing network.  We tell them to select sites frequented by people age 50+.  They then select the individual sites for posting the ads. They may have 10,000 sites in their network.  They place our ads on the sites must suitable for our targeted audience.

The network is in the business of maintaining these 10,000 relationships which that would be impossible for us to do.  And that is why, most companies from General Motors on down, use an ad agency or publishing network to find and establish accounts with individual sites (or radio stations, tv stations, newspapers) because it is not possible for the advertiser to do it.