Or if Facebook is trying to hide the evil, what evil is it they are trying to hide? What will the reach data reveal that is so evil? And if the evil is manifest without reach data, why does it matter?
Basically there is a long thread about Facebook being a haven for the worst qualities of human discussion: insular, xenophobic, and reactionary. This started to get really big during the Cambridge analytica scandel but has continued from there.
In short the argument is that Facebook knowingly allows the aforementioned culture to manifest. In many parts of the world, this has resulted in real world consequences and deaths. That they refuse to divulge the most popular articles is, in this authors mind, a sort of coverup.
Personally I don't fully agree with this assessment. As facebook has become more and more of 'the web', replacing the distributed forums and chatrooms that once dominated, it's also become a reflection of us. Facebook is in a bind no matter what it does--it's either guilty of censorship or misinformation.