... they sure as hell have been rather free and easy in sharing it with others <https://www.techdirt.com/articles/20181219/10370941268/facebooks-latest-privacy-screwup-shows-how-facebooks-worst-enemy-is-still-facebook.shtml>. Seems a bunch of companies have had extra-privileged access to user data on Facebook, and they in turn supplied information about those users back to Facebook. This is how the “People You May Know” feature is able to make links like between different patients of the same psychiatrist, and other things equally dodgy ethically, if not downright illegal (or at least they should be). And this may be how Amazon was able to make a (spurious) association between a book reviewer and the book author, and disallow the review on that basis, just because the reviewer had previously followed the author on Facebook.