My main problem is that they actively try to get their hands on data I don't want to give them. Like, my constantly actualized GPS coordinates, or my browsing history. And that they store it. And process it. And store me into some kind of box.
Oh, and it candidly considers that Facebook, or any other person to whom it sells the data, won't do anything nasty with it. Just serving ads might be okay. But ultimately, they could do much more.
Do you want to know who the gay people are around you ? We could sell you that list ! Do you want to know who votes for your opponent ? We could sell you that list ! And so on.
The day Facebook, or Google, or anyone with a comparable database actually decides to do something evil, it will look ugly. Will they ? Don't know. Don't want to find out.
As a side note, I might pay for Facebook. Or I might have paid. Now, they will just get my money and sell my data anyway, so why bother ?