To quote HighScalability on Scaling Twitter,
Turn your website into an open service by creating an API. Their API is a huge reason for Twitter’s success. It allows user’s to create an ever expanding and ecosystem around Twitter that is difficult to compete with. You can never do all the work your user’s can do and you probably won’t be as creative. So open you application up and make it easy for others to integrate your application with theirs.
In other words, expect your users to be as creative as you are (if note more), and let them compete to build interesting applications around your software.
