As we all know, Facebook lets people update their friends with status updates, and with millions of users, that's a lot of data. Look at the aggregated data over time, and you could see some interesting trends.
The Facebook Data Team recently measured happiness in the United States based on these updates with a metric they call United States Gross National Happiness.
Measuring how well-off, happy or satisfied with life the citizens of a nation are is part of the Gross National Happiness movement. This graph represents how "happy" the nation is doing from day to day, by looking at how many positive and negative words people are using when they update their status: When people are using more positive words (or fewer negative words) in their status updates than usual, that day is happier than usual!
Browse the trends over time, and there's nothing earth-shattering really. You've got dips on the Mondays and peaks on holidays. Although I'm not sure what happened January 22, 2008 to make people so sad. EDIT: It was the day Heath Ledger died [Thanks, Amanda].
Big picture though, I'm sure governments, businesses, organizations, etc would be more than pleased to have something like this when they made a new policy, launched a new product, or started a new initiative.
That's probably why so many are fascinated with the publicly available data coming out of Twitter.