"If you're not paying for it, you are the product."
-- blue_beetle on MetaFilter
I love this quote. It's so true. Facebook is no different than Google or Twitter or broadcast TV or radio - you are there so that someone else can sell advertising. Everything else is just to keep your attention between the ads. (So when The New York Times decides to start charging for its website, that's because it's not doing a good enough job differentiating itself and drawing a large enough crowd, or it's not doing a good enough job with its ad sales or a good enough job monetizing its website.)
And then there are games like CityVille where lots of people can play for free because a small number of people have more money than time and are willing to pay to advance in the game quicker.
A few months ago I listened to the audio version of Free: The Future of a Radical Price. It was a really fascinating book. At the time, the book was available free in a number of formats. Oddly, now $10.80 on Amazon.com. Still, a really great book that makes a lot of good points about how large amounts of money can be made by free.