Bilton’s theory is that Facebook wants to incent people to pay to promote their content, so they show unpaid content to far fewer people. Hunter Walk has other theories (and correctly points out that comparing old data to new could be explained in other ways as well).
Occam’s Razor suggests Bilton is right, simply because Facebook has the incentive to make the free views scarce to increase demand for the paid views. But I just don’t know. As an aside, when I promoted a post back in November, Facebook told me I “had 969x as many views because you promoted it.”
Here’s what I do know – if you really care about Facebook likes, don’t just post your stuff to Twitter and then rely on it being republished automatically to Facebook. In my sample size of one, Facebook penalizes you significantly for that and shows that content to far fewer people.