Earlier quoted context omitted.
If you aren't buying a product, you are the product. That's worth drilling into your head. Yes, that means that User = product from HN's perspective, but I am willing to bet it is a lot more of a complex connection than it is with facebook ;-)
I didn't buy my copy of Debian, yet they treat me better than many companies I pay. I pay for cable, yet I still get advertising. That "not paying means you're the product" is a nice sound bite, but it doesn't actually mean anything.
Paying for media is an interesting exception. Very often for newspapers, etc, you are both buying a product and becoming a product.
But in the end it's a good warning to know the business model of those you get free stuff from.