You are not Facebook's customer. You are their product. You have no expectation of privacy for anything you post to Facebook, whether or not it's "private".
In the same way that "must" can either mean that something is inevitable, or mean that it is imperative.
It's misleading and it shouldn't be used that way. Can't be used that way, even...
You really aren't Facebook's customer - no money is exchanged for services rendered.
In short, because it doesn’t have much substance and may actually be counter-productive.
It's about as "isn't nearly as settled" as the question of whether Santa Claus is real. Just because there's a guy in a red suit at the mall and you got presents every year doesn't mean there's an actual jolly elf flying around in his sleigh handing the best presents to the richest kids. So yes, there's an illusion of "privacy". Your private data is private from your peers, who are just as much a product as you are. But it is never going to be private from Facebook's actual customers.
Market forces are relentless and amoral. This doesn't mean they're immoral, that they're evil. It just means they will not make an effort to be good, either. Facebook doesn't exist to facilitate communication and share cat videos. It facilitates communication in order to serve its actual purpose, which is to collect large amounts of valuable personal data from as many individuals as possible, in order to sell that data to companies that want to sell products to targeted individuals.
The whole purpose of government is to regulate amoral market forces to conform to our moral philosophy.
I agree that government should (and does) regulate amoral market forces to conform to a moral philosophy (although I disagree that it's the whole purpose). But at a certain point, it becomes difficult to manage. Consider the case of "Facebook sells your private messages". Undoubtedly, somewhere in the click-through license that no one ever bothered to read is a provision allowing them to do exactly that, making it clear that what you type into their platform is theirs to resell, whether it's "private" or not. So users agreed to this. For sufficiently abstract interpretations of "agreed". It may not be ethical, but it's certainly legal.
So what does government do about this amoral market force, exactly? Do they try to come up with a rigid, legalistic definition of "private" that businesses like Facebook must abide by? And what is "private", then? My IMs? My age? My friends? The history of what ads I've clicked on before? And how does this government regulation respond to the seemingly infinite imagination and determination of market forces? Like I said, it's like thermodynamics. It's hard to make a law that says no gasses shall pass beyond this seal, except gasses that are permitted by law to pass.
And to be absolutely clear, I'm not saying we should do nothing, or that this is a Good Thing, or that government should just "stay out of the way". I'm saying that it's really, really hard. And sometimes the only way to find the bugs is to run the code.
In the meantime, public hostility to the news is working better than any regulation would have, imho. Facebook went too far and alienated users. It may not hurt them much now, but it's part of a relentless drip-drip-drip that will eventually destroy them. Erosion is also a natural, amoral force.
And until then, we have a solution - don't use Facebook. Don't use social media that you don't completely trust. Yeah, it's annoying and stupid, but it's not as stupid as thinking they're going to respect your privacy if they can make a buck selling it.