Fundamentally I think that if you can only see positive outcomes of anything, it shows you haven't understood it properly. Perhaps there are things without negatives, but I can't think of any off-hand. Yes, Facebook and Google have many benefits, and I continue using them. But they are also a social experiment on a scale that has rarely if ever been conducted before.
Yes, organisations like supermarkets have collected data for years. But Facebook and Google have enough extra data to allow them to learn/guess all sorts of things about you that you haven't told them. And this is made worse by their tracking you on other sites. Technically, this is probably included in the terms and conditions, but I doubt it is known by most of their users, let alone that they have understood it sufficiently to give informed consent. A better analogy for supermarkets would be if carrying a rewards card meant not only that the supermarket knew what you purchased, but also could record and cross-reference any discussion you had within the supermarket, as well as being able to track which library books you borrowed, which music you listened to, where you travelled, who your friends were and what they purchased, and so on. Not what you signed up for? Sorry, it's in the T&Cs.
Another point referenced earlier in this conversation but not noticed is that our information is not solely within our control. Facebook can get information about us from our friends, and even completely opting out doesn't stop this.
Yes, we have sometimes explicitly given Facebook some of our information. But I don't think we fully realise how much information we are giving them that we haven't explicitly chosen to share. And I don't think we fully realise how many different ways they can and do use that information. Fundamentally, privacy matters, and consent to giving out private information should go beyond what information you give to how it can be used.
It's the same as if you order something online. You give your address for a specific purpose, and you consent to a parcel being delivered to your address. You don't consent to door-to-door marketing at that address. Why should it be different for an online organisation like Facebook?
Finally, as I've already said, I have two other concerns that go beyond Facebook: That this rich source of personal information is at least theoretically available to governments, not just a company trying to sell ads, and that innocent actions now may look very different in 20 years. And Facebook won't be forgetting any of it unless it's forced to (maybe not even then).