Twitter‘s new ad platform has been a little slow to roll out. Promoted Tweets are already in the system, but those popular trending topics have only today been penetrated by ads. On Twitter’s homepage, the right-hand column that houses the trending topics now includes an eleventh in the list: Toy Story 3, branded with a prominent yellow button. Clicking the link brings up a search for “Toy Story 3,” and the top result is always a Promoted Tweet...