Via Consumerist, I can't find a source for this other than imgur:
If you can't read it, it's one pig saying, "Isn't it great? We have to pay nothing for the barn." Another responds, "Yeah! And even the food is free." Caption reads: "Facebook and you. If you're not paying for it, you're not the customer. You're the product being sold."
While the cartoon specifically mentions Facebook, this logic applies to media that don't depend on subscriptions or grants or the barely-paid hard work of what-are-essentially-volunteers.
A while back my mother subscribed to House Beautiful, a decorating magazine whose content was about 95% advertising (I exaggerate... but not by much). After two years she decided she didn't have time to read it anymore and didn't renew her subscription, but they kept on calling and offering better and better deals to get her started again. As she put it, they eventually tricked her into getting another subscription by offering it for free along with a book she wanted, and then sent her a version of that book condensed into a pamphlet.
The point of that story is that it was clear that she wasn't the customer, and that House Beautiful's business model depended more on advertising than it did on subscription revenue. Adding another house in the suburbs to its mailing list was an asset, not a liability.
It's what gets me about television, how people think it's free, without ever stopping to realize that if advertisers didn't expect to get at least $1 more than what they spend to produce and run an individual ad (which can cost hundreds of thousands), then they wouldn't do it.
Look around a casino - all that was paid for by money people lost thinking that they'd win. Flip through the TV dial - all that was paid for by money people spent on stuff they otherwise wouldn't have bought.
Not that ads are evil and are always to be avoided, but they definitely don't make something free.