Most of us with Twitter accounts, tweeting 20-30 tweets a week, or a day, offer up a lot of information, not only about ourselves but also about how we feel and think. The hashtags can be organized and pretty much give away a lot of information.
And I'm willing to bet that 99.99% of all tweets are public. This is why the Intelligence Advanced Research Projects Activity (IARPA) is trying to pick up as much information as it can from Twitter to TV to webcams so that it can gauge the mood of society and kinda predict the future.
Seriously, I'm surprised something like this has not already been done. Maybe there is already something like that but we just don't know it yet. I'm sure the intelligence community has a lot of resources to implement this long before.
According to this Nature post, which I recommend you read, not all of it is to predict riots and such. There are also public interest uses for it - like trying to predict disease outbreaks.
It is scary? Kinda. We'll get back to this subject later.