I don't think your Facebook data is right. You're using the "total" count, which includes likes and comments, rather than just shares. While this does measure engagement, it isn't a measure of shares, which is what the data is marketed as. For example, you show 28k likes for the #1 story, but Facebook's data[1] says this:

    share_count: 7023
    like_count: 14341
    comment_count: 7328
    total_count: 28692

[1] http://api.facebook.com/restserver.php?format=json&metho...