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, updating its relationship with its partners, though “we imagine there will be all sorts of other third-party monetization engines that crop up in the vicinity of the timeline”: via blog. Twitter would go on to observe, “We understand that for a few of these companies, the new Terms of Service prohibit activities in which they’ve invested time and money,” but they are moving quickly to deliver Annotations, “so that developers everywhere can create innovative new business solutions on the growing Twitter platform.” This will likely impact Tweetup, Ad.ly, Magpie.