Flight 1549 On Wikipedia: 90 Minutes; 176 Edits
On January 15, 2009, US Airways flight 1549 made a crash landing in the Hudson River in New York after experiencing a "bird strike" (which, as Eddie Izzard points out, is really more like "engine suck").
Fortunately (and surprisingly), no-one died as a result of the incident, but it made headlines around the world. Alongside this, the Twitterati gleefully latched onto another "Twitter beats the mainstream media to the punch" story, helped by the mainstream media shining a light on the way the story rapidly spread through the twitter- and blogosphere.
By pure coincidence, I was on a regular call with a travel-related client (unaffected by the crash) at the time, and I certainly found Twitter useful in passing them relevant information in the minutes following the crash.
Brendan Hodgson and Niall Cook from Hill and Knowlton (a competitor of Thornley Fallis) pulled together a great time-lapse video of the changes over those 90 minutes. If you still need convincing of the speed and power of social media, take 90 seconds to watch this video.
(Hat tip to Dave Jones for drawing my attention to this video)