So-called “Promoted Tweets” will appear when people search for particular terms. Only a single sponsored Tweet will appear alongside search results. The Tweet will appear as long as it demonstrates “resonance” with the audience by being clicked or re-Tweeted. Twitter doesn’t plan to charge companies whose sponsored Tweets don’t generate high resonance. Presumably Tweets with high resonance scores will pay price premiums.
The above is an excerpt from Scott Anthony’s article Twitter Ads Add Intrigue – Scott Anthony – Harvard Business Review on Twitter’s business model.