Twitter is finally rolling out its new promoted tweets system, which places companies’ tweets at the top of users’ twitter timelines. The tweets only appear if a user follows a company that is utilizing the promoted tweets system, and will be clearly marked as ads. Twitter’s explanation to advertisers says that “the Promoted Tweet scrolls down the follower’s timeline as new Tweets appear” and that “advertisers pay only when a user engages.” AllThingsD reports that there will be a limited number of promoted tweets displayed to users.