I've been working with Facebook's ad api since its alpha days, and I can tell you that this problem has always been there. This is not just with Facebook - its

I've been working with Facebook's ad api since its alpha days, and I can tell you that this problem has always been there. This is not just with Facebook - its a many other mobile/display ad networks as well. That is why clients insist on using 3rd party tracking solutions - they trust those numbers better. We are used to seeing discrepancy between reported impressions and clicks ON the ad platform and those reported by 3rd party tools. Its just the way its always been, and I've never heard an explanation that sounded plausible enough to be repeated to a client.