The vast majority has bought into the utter bullshit that advertising makes the web free. This delusion buries not only the fact that we have made a deal with the devil, but also that the deal really sucks. What we traded our souls for we don’t even get. The web would be both cheaper and better if we just paid for what we use straight up. And more importantly, society would be better. I'll explain all of these.
IT'S NOT FREE
We’re not Facebook’s customers, advertisers are (more on this below). But we are the advertiser’s customers, and the cost of the "free lunch" is simply shifted to the price of the things we buy from them. In other words we still end up paying for the full cost of Facebook (and even more, as I'll get to next). Costs may even shift regressively, to advertised products predominately consumed by those with lower incomes, in which case the poor are subsidizing the better off.
IT'S MORE EXPENSIVE...
Not only are you still paying for the full cost of the Facebook product you use, you are paying for all the advertising overhead: the costs of its advertising technology and infrastructure (huge, btw), the agency and creative costs (Don Draper and company have to pay for the hookers and scotch somehow, not to mention what’s-his-name who basically just lounges in his office barefoot thinking Japanese), and the advertiser's big marketing departments (that often outnumber and outspend the people making the product!).
The best minds of my generation are thinking about how to make people click ads. That sucks. – Jeff Hammerbacher, fmr. Manager of Facebook Data Team, founder of Cloudera
So in addition to the original product cost and the ad overhead costs, you are also paying the opportunity cost of an inferior product (as Dennis Curtis points out in the OP) as well as the engineering costs of figuring out how to optimize ad revenue, because that’s what happens when websites have to design to please advertisers over pleasing us, the users. Dalton Caldwell makes this point comparing Sourceforge to Github[1]. As ergo says in a comment[2], “If the new news feed is making their advertisers happy (and bringing revenue into Facebook), then that's what they optimize for.” As jfoster says in a comment[3], “Ad-supported models untie the relationship between UX and revenue.”
Furthermore, our identities and privacy are bought and sold to the highest bidders. And where do the bidders get their money? From us of course! A double whammy! We're trading our privacy for free product? Bullshit. We get personalization? Bullshit. Personalization means optimizing something for me, not optimizing for the advertiser. Again, we're not the real customer. We’re certainly not Google’s[4][5].
WAIT. IT'S EVEN WORSE...
Advertising has us chasing cars and clothes, working jobs we hate so we can buy shit we don’t need. – Tyler Durden, Fight Club
Think of the social costs of advertising. The web is infested with misinformation and manipulation. Beside the lying ads themselves, relying on a revenue stream entirely dependent on how many ads are seen severely affects the moral choices of those who decide what gets produced and how its presented. What are the costs of a misinformed and variously manipulated citizenry, of distortions to the free-market?
Knowledge and discourse are the lifeblood of both democracy, free markets, progress. The web, from the little scammy websites to the big brand ones that so many blindly trust, has a huge influence on who we vote for, what we buy, and most importantly, what we believe.
There is no free lunch, and there is no free web. This "free" ad-"supported" web we have is much too expensive.
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[1] http://daltoncaldwell.com/an-audacious-proposal
[2] https://news.ycombinator.com/item?id=7484075
[3] https://news.ycombinator.com/item?id=7484442
[4] Lloyd made his pitch, proposing a quantum version of Google’s search engine whereby users could make queries and receive results without Google knowing which questions were asked. The men were intrigued. But after conferring with their business manager the next day, Brin and Page informed Lloyd that his scheme went against their business plan. "They want to know everything about everybody who uses their products and services," he joked. - Wired, http://www.wired.com/2013/10/computers-big-data/all/
[5] Google's once famous clean and neutral search results are now cluttered and biased (http://arstechnica.com/business/2013/10/new-banner-ads-push-...).