Twitter is in the process of refining it's advertising service, Promoted Tweets. Specifically, it is trying to expand the types of advertising opportunities for big brands to engage Tweeters. A service service will allow brands to advertise to followers only--rather than to the general public.
Similar to normal "promoted tweets", this new service will appear at the top of users’ timelines upon login. Ultimately, this new web advertising service is a targeting tool for big brands. Normally, promoted tweets generate 3-5% engagement with users, but this new service will help to push that number up.
The service will also help to extend the engagement metrics by ensuring that these particular promoted tweets are accessible to users even if a user isn't online when the ad is originally pushed. Additionally, the ads will extend beyond the Twitter.com website--into other platforms and vehicles for Twitter's service.