Comment by CrendKing

But how to you quantify the effect of the exposure? How do you know a particular sale is converted through some impressions you put on Facebook six months ago, and not from a highway billboard one week earlier? If you can't accurate quantify the effect of each ad campaign with their conversion rate, you have no reason to put more funds into this particular Facebook campaign. So naturally it falls on Facebook to prove to you that their ad socket is actually generating conversion, by not showing the ads to people who is least likely to click through ads when shown.