In a lengthy post on its Web site on Monday Twitter said it will no longer “allow any third party to inject paid tweets into a timeline on any service that leverages the Twitter API.” “Why are we prohibiting these kinds of ads? First, third party ad networks are not necessarily looking to preserve the unique user experience Twitter has created,” the company said on its blog. “They may optimize for either market share or short-term revenue at the expense of the long-term health of the Twitter platform. For example, a third party ad network may seek to maximize ad impressions and click through rates even if it leads to a net decrease in Twitter use due to user dissatisfaction.

Twitter said it employs more than 200 people and plans to grow. To help do that, it is announcing Promoted Tweets. “These tweets will exist primarily in search and then in the timeline, but in a manner that preserves the integrity and relevance of the timeline.”

Twitter said its guiding principles are:

• We don’t seek to control what users tweet. And users own their own tweets.
• We believe there are opportunities to sell ads, build vertical applications, provide breakthrough analytics, and more. Companies are selling real-time display ads or other kinds of mobile ads around the timelines on many Twitter clients, and we derive no explicit value from those ads. That’s fine. We imagine there will be all sorts of other third-party monetization engines that crop up in the vicinity of the timeline.
• We don’t believe we always need to participate in the myriad ways in which other companies monetize the network.

It will be interesting to see how this plays out.