On the eve of its inaugural developer conference, Twitter is having to explain itself to third-party developers becoming increasingly nervous that new features being added to the micro-blogging site will gut their businesses.

In an e-mail sent over the weekend, Ryan Sarver, director of the Twitter platform, tried to calm developers by explaining Twitter's plans to acquire Atebits, announced Friday. Atebits makes the popular Tweetie application for posting messages on Twitter via an Apple iPhone.

The acquisition announcement left developers wondering if their applications would eventually become unnecessary as Twitter builds or acquires its own features. Making matters worse, Twitter labeled "official" a mobile app it's building in conjunction with Research In Motion for the latter company's BlackBerry smartphone.

In his e-mail, which first appeared in the San Francisco Chronicle, Sarver said that Twitter's decision to launch its own iPhone app was intended to end the confusion caused by the many Twitter applications on Apple's App Store for the iPhone. The number of available applications made it difficult for new iPhone users to find what they wanted, and many would just give up.

"This means that we were missing out an opportunity to grow the user base which is beneficial for the health of the entire ecosystem," Sarver said.

He went on to say that the company plans to continue adding new features, and acknowledged "the potential to upset a company or developer that may have been building in that space and they then have to look for new ways to create value for users."

"My promise is that we will be consistent in always focusing on what's best for the user and the ecosystem as a whole and we will be sincere and honest in our communication with you," Sarver said.

The use of third-party developers to quickly expand functionality within a software product or Web service is not new. Companies that have been successful with such strategies include Microsoft, Google, Facebook, and many others.

In Twitter's case, developer support has helped the site soar. In March, the number of visitors reached 9.3 million, an increase of more than 5 million visitors, or 131% from February, according to ComScore. However, dependence on third-party developers often leaves companies walking a fine line between adding features themselves without alienating the developers they need for continued growth. Twitter's position is particularly precarious, because it also needs to find ways to make money to satisfy investors. The company is expected to announce how it expects to become profitable at the Chirp developers conference, April 14-15 in San Francisco.

One investor watching Twitter closely is Union Square Ventures. Fred Wilson, a Twitter director and partner at Union Square, pointed out that some of the most popular services on Twitter came from third parties, including access from mobile phones, photo sharing, and search.

"Twitter really should have had all of that when it launched or it should have built those services right into the Twitter experience," he said in his blog.

Going forward, Twitter will have to incorporate more features within the site, while also finding a way not to lose the developers that add services it may not even think of, Wilson said.

"It's time for Twitter and its developer ecosystem to work together to create entirely new things that will shape the Internet in the coming years," Wilson said.

However, that's likely to take time and require lots of close communications with developers. Indications of developer concern can be seen in the comments on Sarver's e-mail.

"With these recent moves and announcements you're starting to realize (for the first time) lots of developer's fear: 'what if Twitter makes that same feature I'm working on?'" developer Arnaud Meunier wrote. "This situation happened on a lot of other platforms before, and I guess we all knew it was going to happen here, soon or later. The only questions were when and who. One of the question is still opened. I hope we'll find answers @chirp."