Twitter has grown by leaps and bounds and is amongst the top social / news / sharing sites in the world today. People across the globe are using it to share short messages not just at a personal level, but also to tackle real-world events. There are many instances of how twitter was helpful during crises situations like wars, storms, floods, and more. So, twitter is no doubt a useful tool.
Back in 2009, when twitter was on it's rapid growth path, a team of (data) analysts looked at the data from twitter, and put forth some interesting facts and observations: this infographic below (courtesy of InformationIsBeautiful.net) shows what twitter is (rather, was like) then. [Things may have changed since, and hope that these folks are kind enough to provide a revised infographic!]