Twitter on Monday banned outside advertising in the stream of messages at the hot microblogging service. Twitter's own 'Promoted Tweets' advertising service will remain in place, allowing companies and others to place 140-character-or-less messages known as 'tweets' at the top of a page of search results. '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,' Twitter chief operating officer Dick Costolo said in a blog post. 'As our primary concern is the long-term health and value of the network, we have and will continue to forgo near-term revenue opportunities in the service of carefully metering the impact of Promoted Tweets on the user experience.' A prime reason for the ban is to prevent ads from marring the 'unique user experience' at Twitter of real time streaming comments, according to Costolo. '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,' Costolo said. Twitter, which has seen explosive growth since its launch four years ago, unveiled in April its Promoted Tweet plan to use advertising to turn its massive popularity into profit. 'Promoted Tweets are ordinary tweets that businesses and organizations want to highlight to a wider group of users,' Twitter co-founder Biz Stone said at the time. Twitter acknowledged the outside advertising ban would be a blow to some software makers behind applications crafted to work with the microblogging service. 'We understand that for a few of these companies, the new Terms of Service prohibit activities in which they've invested time and money,' Costolo said. '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.' The new terms of service will not prevent celebrities or others from getting paid to make endorsements or other promotional commentary in their tweets. 