Case of Build an extensible Platform, Provide good support and Developers will come. Twitter now has One Million Registered Twitter Apps.

As an ecosystem, we’ve just crossed one million registered applications, built by more than 750,000 developers around the world. This is up from 150,000 apps just a year ago. A new app is registered every 1.5 seconds, fueling a spike in ecosystem growth in the areas of analytics, curation and publisher tools.
Investors and businesses are taking note. Since December 2010, more than $500 million has been invested in ecosystem companies, and more than a billion dollars has been paid out in acquisitions. This level of investment is indicative of the opportunity for developers and entrepreneurs to build successful businesses as part of the Twitter platform.