As with many discussions, there are multiple sides and perspectives. I agree with your assessment of Facebook. My wife uses it for that purpose. However, also realize that various aspects of information are "harvested" for the purposes Nathan described. This goes on all the time. Any time we engage the internet for any purpose, there is something about us that can be harvested, such as our email address, content of messages we post in forums or other publicly visible venues, the sites we visit that leave "cookies", and on and on. These things can be, and often are, tracked by somebody depending on the purpose for collecting the information.
So the points brought up in this thread are not mutually exclusive. Rather than supporting a conspiracy theory, I am simply saying that we should be careful as to what we say and do on the internet. I would not be surprised if somebody brings still more information into the discussion that would counter what I am saying, such as the relative safety/anonymity of using the Tor network (what some call the "dark net"), but even there, we still have to "come up for air" to access anything we would be normally using on the internet. The internet is a big space and there are lots of people involved, some for social reasons, some for rather nefarious reasons. So, in actuality, it is all true.
Edit:
Thinking about what I wrote here, it might be helpful for me to clarify about "harvesting data". People I have known in the sales and marketing area maintain lists they call "contact lists" - people to contact who they consider potential customers. These contact lists are (were?) consider quite valuable because they serve as valuable tools to generate sales, which in turn generates their income. Typically "harvesting" is all about creating and selling these contact lists to be used for mass emailing, targeted marketing, and such. This is different from hacking your private data to use for identity theft. What I am referring to in this post is the harvesting to build such contact lists, rather than the more nefarious hacking to acquire personal data for identity theft, though that certainly occurs also.
I found it quite interesting to read some of those books on building an internet business because though I have no interest in doing so, I learned a bit about how this stuff works. Aside from having a product (or reselling somebody else's product) the big issue such books address is how to get yourself out there so people know who you are and what you are doing. There is a whole effort and skill set involved in how to get your business at the top of a google search, how to get your business mentioned and linked on other people's web sites and how to build that all-important email contact list. That is big business because there is a lot of money (relative to an individual running a business from home as well as for larger businesses) at stake when it comes to building your customer base, and that is what this type of "harvesting" is all about.
Your contact information is bought and sold all the time, and collecting and maintaining such data is a business with a potentially healthy income of its own. We get junk mail in the regular mail all the time, just as we do get junk emails advertising all manner of stuff. In terms of how the information about who to send this stuff to, the mechanisms are not so surprisingly identical. Have you ever noticed that soon after you start looking around online for certain types of products or regularly visit special interest web sites and forums, you start getting email related to those interests or that once you are identified as a "senior citizen" because you start collecting Social Security or otherwise start getting some sort of benefit such as a reduced rate on certain insurances because you no longer drive to work, that you start getting ad mail directed at older people?
Tony