Although in beta and only open to VIP advertisers that are invited, Twitter has finally released their ad platform called Promoted Tweets. The general format is that you can use tweets you’ve created or that have been retweeted by someone else as “ads” that are promoted in certain environments. The way the ads are served are analogous to both search and contextual advertising in AdWords, where you pick keywords that are searched on to have your tweet shown or it is matched to a stream of tweets given the contextual nature that you choose in your account. You pay on a cost per engagement which include clicks, favoriting, retweets and replies. There is also Promoted Account where you can essentially buy followers and Promoted Trends where you are shown on hashtags.