> It is a widely assumed belief that I have never seen backed up with hard facts.

‘Privacy’ or even ‘Data’ are not even remotely well defined enough for those statements to have a clear meaning. However, many people have felt betrayed when using Facebook.

Some of it — potentially the majority of cases — have to do with the fact that the service is new, and the lack of understanding led to many context collapses: there are many situations to consider, but all involve three people knowing each other, one having authority over a second, but not a third. Classic case involve a teenage child, a parent and a common friend or relative. The relative reveals inadvertently something. This is far more disempowering to non-users mentioned by friends, or whose photo was taken.

Facebook was openly careless about those cases, manu legitimate — Mark Zuckerberg went up to publicly justify his disregard by saying that people shouldn’t have things to hide to their friends, a callous statement if there is any.

There are many more problematic cases, where the person feeling betrayed was attentive and knowledgable, but Facebook changed the way they handle privacy without clear warning, and something that wasn’t became visible. This is the case of profile pictures, for instance. You have important social information there; some people assumed that their photo with their significant other would remain as private as their set it, but that changed, without recourses. There are many more similar issues, related to poor explanations of updates.

There is no way the wordings chose for most updates came form considerate product managers.

There are more examples, generally to the overall attitude: ‘better ask for foregiveness than permission’, rephrased as (and plastered all over their headquarter as essential values): “Move fast, break things”. When it comes to privacy, you can’t get the cat back in the bag.

Company motto, public statements by the Founder-CEO, presentation of privacy-impacting updates… that’s plenty of proof for who would take five minutes to care looking.

I know for a fact that people at Facebook care about those — just they haven’t cared enough, repeatedly; and those cases where well documented.

You just answered your own question. It is generally accepted that Facebook and Google derive a large source of their revenue from advertisement, which is in such high demand at these companies precisely because they control so much of your personal data.

It isn't so much of a leap of reasoning to think that a company willing to sell some information about you isn't willing to go further than you'd like them to. Again, it's not that we're worried about them 'leaking' it - we're worried about how they're going to sell and make money off of it.