Twitter on Tuesday introduced a new service called Promoted Tweets, which will let companies send ads in the form of ordinary messages, according to a blog posted by co-founder Biz Stone.The introduction is an important step for the company, since it represents the first step towards turning the microblooging service into a money-making venture.Users will start seeing paid messages, which will be labeled "promoted," at the top of some Twitter.com search results pages, according to Stone. Initially, up to 10 percent of users will see the promoted messages. Twitter will work with Best Buy, Bravo, Red Bull, Sony Pictures, Starbucks, and Virgin America, to roll out the first ads, he said.Twitter roll out the Promoted Tweets in several phases, with Tuesday's announcement marking the first. Before Twitter develops the service further, the company wants to get a better understanding of the "resonance" of Promoted Tweets, the user experience and advertiser value, Stone said.Future changes to the ad service could include showing the messages in users' message timelines, he said.The ads will have to fight to get user attention. Twitter will attempt to measure whether the advertising messages interest users and stop showing those that don't, according to Stone. They will have all the functionality of a regular message, including the ability to reply, retweet and favorite.On Tuesday, Twitter's COO Dick Costolo will be talking about the offering in detail at the AdAge Digital conference, according to Stone. Costolo and CEO Evan Williams will further discuss Promoted Tweets and what it means for Twitter developers at Chirp, Twitter's developer conference, on Wednesday, he said.