One January afternoon, I saw a remarkable event unfold on Twitter.
It was around 3:30 p.m., and I was sitting at my desk. A few people I followed on Twitter suddenly began sending out short text messages (called tweets) about a plane crash. The plane had apparently gone down in New York City, right in the Hudson River. New Yorkers in nearby buildings had seen the crash, or spotted a plane floating in the river, and were sharing scraps of information in the short messages that Twitter allows.
I checked CNN and Google News. There were no official news reports. Yet people on the ground were reporting a disaster. What was going on? ...