How is this possible, you might ask? After all, there are tens of thousands of web services and applications out there, and many of them are successful. Why is there so much news about Twitter? I've been trying to figure this out myself, and came to the following conclusion.
Twitter has a notoriously small feature set. The service itself - as I've explained more eloquently in a lengthy post about Twitter's simplicity - is little more than an API for a simple one-to-many short message broadcast system. Because of it, in the beginning it had to overcome misunderstanding from many users, bloggers and experts, who simply didn't get it. However, once it reached critical mass, it started reaping the benefits of being simple and, for lack of a better word, featureless.
Because of its simplicity, Twitter offers a huge, untouched foundation to build upon. Every feature it lacks, such as groups, adds (perhaps even exponentially) to the number of the applications that can be written for it. And every single one of those applications is a free ad for Twitter.
Sites such as Mashable simply cannot not write about these apps, because they're fun and useful and make Twitter a perpetually interesting topic. Furthermore, the info on those applications comes from various sources and different people, which make things much more interesting for journalists than receiving a press release from Twitter every now and then.