The AP has a story today on a division of the CIA that sits around and reads tweets all day. Like, 5 million tweets a day. It's called the Open Source Center and is referred to within the intelligence community as the "vengeful librarians."
From Arabic to Mandarin Chinese, from an angry tweet to a thoughtful blog, the analysts gather the information, often in native tongue. They cross-reference it with the local newspaper or a clandestinely intercepted phone conversation. From there, they build a picture sought by the highest levels at the White House, giving a real-time peek, for example, at the mood of a region after the Navy SEAL raid that killed Osama bin Laden or perhaps a prediction of which Mideast nation seems ripe for revolt.
They've been following Twitter closely since the 2009 protests in Teheran and were able to predict the overthrow of Mubarak in Egypt.