There's a big difference between Marketing and Spying.....
Even without snooping, the amount of data they can legitimately look into and aggregate gives them so much insight into our patterns. It's not about Facebook but a grocery chain but I remember reading a book (I believe the title is Numerati) where a father complained that they started receiving pamphlets for baby care products but then later sent the store an apology letter. It turns out their algorithm correctly predicted the daughter is pregnant - even before she found out! Just imagine how much more data FB have access to.