Was this a breach in trust to Facebook users? I think undoubtedly yes.
And was there a breach of a the Terms of Service by companies taking all this data and using it for non-academic purposes? Yes there was.
So the type of breach seems to be a worthwhile distinction to make.
What's interesting about this is the fact that the same data is shared with many third-parties, with proper "consent", and users not understanding what's really happening. Calling this a "breach" has the slight unintended side-effect in the public by promoting the idea that this company received a different dataset than other partners, which is not the case.
There's a legal concept of 'waiver' meaning that even if something is prohibited in a contract, but the parties don't enforce that part, then that part is later not enforceable. Facebook was fully aware of this behavior, chose not to enforce the ToS, and therefore it waived that clause. Therefore no breach.
How naive is the average person? The purpose of facebook is to gather this information, hence why its offered as a "free service".
Frankly, I don't understand why the stock is going down, facebook is fulfilling its core mission: Get private information on millions of people and package that information for sale to its clients. If anything CA situation should show how FB is fulfilling its core mission.
The fact that the public is now waking up to this is not a breach, its simply casting a light on what has always existed.
The public waking up to this breach and the costs being exposed are probably a huge part of why the stock is dropping. Facebook's continued profitability and success is dependent on its users not understanding how their data is being used. And now "everyone" knows, so the secret is out and hopefully Facebook can't get away with this going forward.