The site that played a major role during the terrorist attack on Mumbai…

Concept: Which medium dominates the reporting of a live, fast-moving public emergency is decided by how its messages travel. Microblogging — very short messages that are public by default, pushed instantly to followers and retrievable by anyone through a shared tag — turns every bystander into a live news source. A profile-and-friend-graph social network routes updates mainly to the people a user has already connected with, and a transactional commerce site carries no message-broadcast channel at all.

Application: The Mumbai attacks of 26–29 November 2008 unfolded over roughly sixty hours across several locations at once. Eyewitnesses, trapped hotel guests and journalists posted short updates tagged #mumbai; the stream ran at thousands of messages an hour and was quoted directly by international broadcasters and newspapers while the operation was still under way. That open, real-time, searchable stream is what made Twitter the site that played the major role.

Contrast with the other services:

• Orkut carried its activity inside member profiles, scrapbooks and topic-based communities, so discussion stayed within those communities instead of forming one public, searchable live feed.

• Facebook in 2008 delivered its news feed chiefly to a user's confirmed friends, so updates travelled along private friend graphs rather than acting as an open newswire.

• Amazon.com is an online retail catalogue for listing, ordering and reviewing products, and carries no message-broadcast or news-sharing channel.

Cross-check: Contemporaneous reporting from The Guardian, Forbes and ABC News on 27–28 November 2008 all identified the #mumbai feed as the fastest public source of updates, and Indian officials publicly appealed to users to stop posting live details of the security operation — an appeal that only makes sense against a stream large enough to matter.

Answer: Twitter.