More than 70,000 applications make it possible to post photos, shorten URLs, and do other useful things on Twitter. They are a primary reason why the micro-blogging service now boasts 50 million Tweets a day — up from 5,000 in 2007.

Twitter doesn’t pay the developers who created all this utility and, by extension, the company’s unassailable position. Instead, these folks monetize their work by charging users for the app and/or by selling advertising. But they’re starting to wonder where their next paycheck will come from, for there’s a new competitor on the scene – Twitter itself.

Now that it has achieved ubiquity, Twitter is looking for revenue, and spent part of its venture capital hoard last week to buy Atebits, which makes Tweetie for the iPhone and Mac. It also said it is creating BlackBerry’s official Twitter app.

“The time for filling the holes in the Twitter service has come and gone,” wrote Twitter investor and board member Fred Wilson in a blog post cited in the New York Times (4/11/10). “Twitter really should have had all of that when it launched or it should have built those services right into the Twitter experience.”

In other words, you take a great risk when you’re an outsider “filling the holes” in someone else’s product — that is, writing code for a corporate-owned platform. Once you’ve helped build the service into preeminence, the corporation may try to buy you (not bad) or bury you (bad). It may also leave you alone, but the likely choices have narrowed. Happens all the time. As cited in the Times report, outside developers helped make Microsoft ubiquitous by creating tools to enhance its operating system. Then the company re-created these tools and built them into Windows, and the outsiders were shut out.

Twitter, a free service, plans to introduce paid accounts for businesses, including a tool for managing corporate tweets with multiple authors – inspired by a similar program developed by an outside firm called CoTweet, which will now have to compete with Twitter for sales.

How should developers orient themselves on this changing landscape? Wilson recommended that they focus on such add-on services as business tools, analytics, or gaming. The new terrain will be further explained and explored at the first-ever Chirp conference for Twitter developers, which starts today in San Francisco.

See: http://chirp.twitter.com/index.html