This is something that I find *extremely* troubling about targeted political ads and info segregation:

"Targeting made tracking the actual messaging that the campaigns were paying for impossible to track. On Facebook, the campaigns could show ads only to the people they targeted. We couldn’t actually see the messages that were actually reaching people in battleground areas. From the outside, it was a technical impossibility to know what ads were running on Facebook, one that the company had fought to keep intact."

https://www.theatlantic.com/technology/archive/2017/10/what-facebook-did/542502/