That's part of the reason I asked. Based on the ridiculously bad suggestions I'd often get (from search, netflix and the lik, etc.), it all seemed a bit too much hot air.
But a few things made me (sort of) change my mind over time, and worry more actively:
1. the ability to properly use the data available is a skill in itself. The fact that this 'new, non-private' internet is still relatively young (really just a decade, post-smartphone I'd say) could mean that most companies/governments simply didn't know yet how to properly use it. Considering the many examples of the slowness with which organizations tend to adapt, that doesn't seem unlikely.
2. There's a huge difference between the data available, and the targeting achievable pre-smartphone, and post-smartphone. On top of that, I distinctly remember that throughout most of my youth (so up until a few years before world-wide Facebook) it was still very much a 'default' attitude to be very careful about your personal information online. Not quite the "don't go into chatrooms" of my childhood, but still the "don't use your real name, ideally, on <national social network>".
3. We've made quite a number of advances already in storing and analyzing 'big data', from what I understand. And a decade or two of research on this new world of personal internet data must have led to some significant advances, no? I mean, so far we've gotten really good at manipulating people with whatever new technology entered the scene. Just not right away, necessarily.
4. from personal experience, and conversations with people who do/did targeted advertising on Facebook, it really does seem shockingly easy to target specific messages to specific groups. I find it hard to believe that this does not fundamentally change the whole game of manipulation, compared to things like TV and newspapers. Hell, 'old fashioned' direct marketing / door to door sales is perhaps a precursor of all this. I remember reading quite a while back how advanced the techniques were to decide what neighborhoods to visit or snailmail-spam for particular products or services. If companies were willing to spend the amount of money necessary for that, it must have some effect. Being able to target essentially large parts of the entire world in that same way has got to be at least somewhat more powerful, which is scary enough maybe.
I'm not sure of any of this, though. Just thinking out loud and very curious to hear what others think (although ideally I'd also love some suggestions of where to find the best research/articles on the matter).