A short but interesting article in the WSJ goes into the use of "nudges" and social pressure to encourage people to modify their behavior. The basic idea is that people don't always behave rationally or in their best self-interest. While this wasn't big news to the rest of the world, apparently it is for many mainstream economists, who continue (or did, until recently) to believe that markets are always efficient because people will always carefully weigh choices and make the best one. One of the bits that stood out for me was the theme of social pressure being used to modify electrical use in Sacramento. By making neighbor's power use known, utility customers will actually lower theirs to meet or beat their neighbor's. When we think people are watching us, our behavior is quite different, it turns out. A lot of this is covered in "Nudge", a great read on the subject.
This kind of feedback, along with ordering choices and playing off people's tendencies to overvalue free things, can be useful tools in designing user experiences. We've been exploring this at Chartbeat (betaworks), and I've been wanting to leverage this more in fund-raising at my children's school (where we already have had some success using social media).