Earlier quoted context omitted.
I'm not sure why this post is getting down-voted aside from people not liking what it says (perhaps how it says it?). Nothing it stated was factually inaccurate or unjustified. You really aren't Facebook's customer - no money is exchanged for services rendered.
I’m guessing because the expectation of privacy thing isn’t nearly as settled as the GP makes it out to be, and depends partly on how the public views such interactions — but the entire rant is deeply based on that point, which it aggressively and baldly asserts. 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.