This slideshow made me laugh.
I've been a Twitter user now for two years, and I'm intrigued by its success.
You can attribute its success to its incredible simplicity, leaving the wider community to develop applications and 'clients' that anyone can chose to adopt or ignore.
You could attribute it to the character limit, meaning that no-one has to worry that they have to put much work into Tweeting (something that holds blogging back to this day), yet also short enough that clever-clogs can innovate and play games with the limitation.
You could attribute it to the ingenious (and incredibly obvious... with the advantage of hindsight) tweak to instant messaging. Take something that is already incredibly popular, but make it more public, more 'many-to-many'. Read more