I think one reason it succeeded is that it fills a gap that was missing before in the online world: a simple way to communicate what was important to you no matter how trivial it is to everyone else. It also makes it easy to check what is going on without slogging through tons of pages.
I also think it was the extensible nature of Twitter. I wrote an article that highlights this: http://accentuate.me/blog/?p=46
I also think it was the extensible nature of Twitter. I wrote an article that highlights this: http://accentuate.me/blog/?p=46