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Twitter chief executive Ev Williams tried to assuage developers a week after the company made its first acquisition of a major iPhone app, threatening competing products created by third parties.

Last week the company bought Atebits, the maker of a popular iPhone app called Tweetie. It marked a major sea change in the way the company treated its ecosystem. For a few years, Twitter had kept its own properties relatively spare and had instead relied on outside developers to create apps and sophisticated clients for reading tweets.

“I know this is a controversial decision because there were Twitter apps on these platforms,” Williams said. “They’re fantastic and we love the innovation there. But when we did the research, we found that we were really underserving users. We had to have a core experience on these platforms like we do on the web. We’re failing the ecosystem because we’re not getting nearly as many people started and engaged.”

He showed a video of a user test with a young woman who couldn’t find an iPhone app to read tweets. She was confused and seemed to give up quickly.

“There is always a tension between platforms and application providers,” Williams said. “What’s more important is what we’re trying to build and where we’re going. Where we make acquisitions isn’t a key factor. Acquisitions should be good news to the developer community. Hopefully it doesn’t alienate them. Hopefully it reaffirms their commitment.”

He tried to assure developers that the company would continue to support many types of products.

“There is a fundamental philosophy that hasn’t changed with Twitter. We believe in openness and a diversity of ideas and that is not changing. We believe in the power of innovation to create new value for users.”

Williams illustrated the history of Twitter, pointing out all the times over the past few years that developers had added to the ecosystem. He mentioned Twitterific and Twittervision, a visualization of tweets on a map. Developers make 3 billion requests to the company’s application programming interface a day and the company now has 175 employees.