Adding to this, if I comment on FB about any other member and use the @-mention feature, but they're not authorized to see the original post, they won't be able to read what I wrote or what anybody else did either. Compare that to G+, where a +-mention actually grants access for the post to the mentioned user whether they originally had it or not.

Imagine planning a surprise birthday party, where someone +-mentions them, even by mistake. So much for surprises!