Twitter on Monday said that third-party ad networks will not be allowed to use the company's API in order to include paid tweets in the Twitter timeline.

Twitter on Monday said that third-party ad networks will not be allowed to use the company's API in order to include paid tweets in the Twitter timeline.

"Aside from Promoted Tweets, we will not allow any third party to inject paid tweets into a timeline on any service that leverages the Twitter API," Dick Costolo, Twitter chief operating officer, wrote in a blog post. "We are updating our Terms of Service to articulate clearly what we mean by this statement, and we encourage you to read the updated API Terms of Service to be released shortly."

Promoted Tweets, which , displays advertiser-backed tweets atop search results. The effort is the micro-blogging's site's first foray into traditional online advertising, and started with several partners, including Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America.

Twitter said at the time  and again in Monday's blog post  that the ads would only display to relevant users; those searching for Starbucks mentions, for example. Twitter said it wants to keep it that way, and that it doesn't trust third-party ad networks to serve up ads in a similar manner.

"Third-party ad networks are not necessarily looking to preserve the unique user experience Twitter has created," Costolo wrote. "They may optimize for either market share or short-term revenue at the expense of the long-term health of the Twitter platform."

If Twitter becomes too cluttered with useless ads, people will leave the service, and Twitter will be done, he continued. "Third party ad networks may be optimized for near-term monetization at the expense of innovating or creating the best user experience."

There is, of course, also money. "Twitter will bear many of the support costs associated with any third-party paid Tweets, as Twitter receives support e-mails related to anything a user sees in a tweet stream. The third-party bears few of these costs by comparison," Costolo wrote.

So what's an advertiser to do? Costolo said the company's Annotations effort, which will allow people to add metadata to their tweets and was announced at this year's Chirp conference, is still a go.

"When Annotations ship, there are going to be many new business opportunities on the Twitter platform in addition to those currently available," he said. "We know that companies and entrepreneurs will create things with Annotations that we couldn't have imagined."

Costolo acknowledged that the new policy will prohibit activities in which companies have invested time and money. "We will continue to move as quickly as we can to deliver the Annotations capability to the market so that developers everywhere can create innovative new business solutions on the growing Twitter platform," he concluded.