> I don't think the problem is what do they actually do with all those data, but what could happen with it.

This is FUD, plain and simple. While I don't advocate sticking one's head in the sand and refusing to predict or even speculate, it's easy to do too much predicting and speculating (and very prone to bias), it's easy to spread paranoia, and it's no basis for living a life and especially it's no way to decide moral or criminal issues. I could go on a murderous rampage tomorrow (perhaps not physically possible as determined by the laws of physics making up my brain but "possible" in the looser sense of it being a member of the state-space of world-configurations). Why don't people I know think this is likely? Largely due to trust that I've built up. Someone once asked me what they could do to gain my trust back when they broke it, I said that it's largely a matter of not doing certain things for a long enough period of time.

And so the problem is heavily dependent on what Google, Facebook, et al. end up actually doing with the data. What they could or might do has little relevance, what they're likely to do has significant relevance but still not as much as what they actually do. Thus far, they've been awesome about either not doing evil things with it or at least not letting me know about their evildoings. Their trust is not yet broken with me, unlike with some other companies I can think of (and correspondingly don't deal with at all).

Therefore I continue to use their products and let them collect and infer as they please. I don't exactly make this easy for them either since I too have "unusual" browsing habits in the form of NoScript, AdBlock, multiple browsers, shared, dynamic IPs and occasionally Tor. I didn't even join Facebook until a couple months ago on a "Let's see who friends me" curiosity (so I don't friend anyone myself). I'm sure there are plenty of Evercookies on my system I'm unaware of though and my full Firefox UserAgent is pretty unique. So they likely have a decent profile of me that they just don't reveal in a straightforward manner. They could also form a good picture from my comments here and elsewhere. (I don't think they have much ad preference data on me except perhaps that I hate ads.)

Though that brings me again to the issue that it can be worth speculating over what is likely to happen that will cause the trust to be broken and if I should be avoiding certain classes of actions that typically lead to harsh repercussions later. For example, suppose you have data showing it's quite likely that lending large sums of money to friends leads to the destruction of the friendship sometime later. Maybe your friendship will be different, but if that outcome is quite likely, you'd probably be better off not taking the risk and in this case acting on facts apart from what the other person has actually done is useful.

Even if these companies break their trust with me in the future, the most likely ways it will be broken do not seem to have incredibly unfortunate outcomes for me. (I can of course think of other quite nasty outcomes as well as anyone.) Similar to how, for example, a friend stealing something petty from you doesn't often cause great financial loss or inconvenience but the trust is shattered nevertheless. Or there may even be actions taken that I never signed up for or wanted and probably would have said "This will violate my trust if they do this" had 'this' been told to me before they did it, yet those actions actually end up being very useful to me and so it doesn't break my trust. I think this is actually how a lot of technological progress has been for a lot of people. They're taken on the ride from the status quo whether they like it or not, but in the aftermath they often like the new place better.