Andrew Seigner, left, and Robert Manson developed an iPhone app, Heypic.me, that uses Twitter data for posting photos

It was the beginning of a beautiful relationship.

Twitter made it easy for programmers outside the company to build 70,000 applications that made the microblogging service more usable. Without them, people would not be able to post a photo, shorten a URL, monitor several Twitter accounts at once, easily use the service from a cellphone or search for people to follow.


Because of that, Twitter grew so fast that no me-too company could mount a serious challenge. People now write 50 million Twitter posts a day, up from just 2.5 million at the beginning of last year and 5,000 in 2007.

The outside developers did it all at no charge because Twitter allowed them to make money from advertisers or Twitter users willing to pay for apps. These programmers — who will gather this week in San Francisco at Chirp, Twitter’s inaugural developer conference — are starting to feel that life is getting a little more complicated.

Serious tension was starting to develop long before the conference, where Twitter is expected to announce ways it will make money, which could include advertising or paid accounts for businesses.

Developers fear that if Twitter’s engineers build the same features that they have, Twitter could transform overnight from generous benefactor to arch competitor to their start-ups.

The tension is becoming more acute as Twitter matures and develops the resources and desire to buy or build its own version of some of the outside apps. On Friday, Twitter announced that it had acquired Atebits, which makes Tweetie for the iPhone and Mac, and that it had worked with Research In Motion to build an official BlackBerry app. That has other start-ups that make mobile Twitter apps wondering if there is any room left for them.

“When we launched, Twitter was incomplete, so developers rushed to fill those holes, but eventually we’re going to have to build a lot of features in because they should be there,” Evan Williams, Twitter’s co-founder and chief executive, said in a recent interview. “We want to set those expectations.”

Fred Wilson, the Union Square Ventures partner who invested in Twitter and serves as a director, echoed that sentiment in a blog post last week that immediately put many developers on edge. “I think the time for filling the holes in the Twitter service has come and gone,” he wrote. “Twitter really should have had all of that when it launched or it should have built those services right into the Twitter experience.”

Pete Karl is an engineer who builds Twitter apps at the Digital Influence Group and his own start-up called Lion Burger. “I’m waiting for the other shoe to drop,” he said. “Before, I think developers had the upper hand. But now it’s time for Twitter to try and make some money, and I think they want to create a situation where the scales are tipped more in Twitter’s favor.”

For the technology industry, this is a familiar story. Giving away technical secrets to those who want to use your product to build their own might seem counterintuitive to most companies, but it is a common path to success in the tech world. Microsoft, Apple, Google and Facebook have all, to varying degrees, opened their platforms this way.

It can be a stunning source of growth.

“Embracing the openness leads to ubiquity, and if you can embrace that ubiquity, you can make money,” said Mark C. Stevens, a lawyer for start-up companies at Fenwick & West in Silicon Valley.

But invariably, the provider and the developers bump against each other. “That’s how it always is when you develop for a corporate-owned platform,” Dave Winer, software developer, blogger and visiting scholar at New York University, wrote last week. “You have to make peace with that reality before you write your first line of code.”

Microsoft, for example, became ubiquitous in large part because of all the tools, like memory managers, that outside developers built. Once Microsoft built memory managers into Windows, those start-ups became irrelevant.

Twitter has been unusually free about letting developers tap into its data and technology, through what is known as an application programming interface. Sometimes it gives developers certain tools, like geolocation for Twitter posts, before it uses them on its own site, and developers can use the data and create a site or app without signing any contracts or even alerting Twitter.

“The problems we’re solving are so big that we need a lot of people working on them and we need to give them the same level of access,” said Ryan Sarver, the director of platform at Twitter.

If developers build something Twitter wants, the company has three options — let it exist separately, create its own version, or buy the start-up, as Twitter did in 2008 with Summize, which created a Twitter search engine, and last week with Atebits.

“For every platform ever, it’s a question of what should be left up to the ecosystem and what should be created on the platform,” Mr. Williams said.

Developers are rapidly learning that truth. Heypic.me is an iPhone app for posting photos to Twitter and to a Web site that shows Twitter photos on a map of the world.

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