From The Atlantic:

How stable is China? What are people discussing and thinking in Pakistan? To answer these sorts of question, the U.S. government has turned to a rich source: social media.

The Associated Press reports that the CIA maintains a social-media tracking center operated out of an nondescript building in a Virginia industrial park. The intelligence analysts at the agency’s Open Source Center, who other agents refer to as “vengeful librarians,” are tasked with sifting through millions of tweets, Facebook messages, online chat logs, and other public data on the World Wide Web to glean insights into the collective moods of regions or groups abroad. According to the Associated Press, these librarians are tracking up to five million tweets a day from places like China, Pakistan and Egypt.

The Atlantic: How The CIA Uses Social Media to Track How People Feel

See also: Predicting the future with Twitter.