“Actions are easy to confirm. Facebook says 10 people commented on a post? I can count that. There were 20 purchases? I’ll check my records (though there are some holes here). Clicks and landing page views, while your records won’t match up 100%, can at least be checked against some verifiable form of reality.

But reach? You have to take Facebook’s word for it. And how they define it can significantly change how it’s reported.”