Get ready to see plenty more hashtag prompts during your favorite shows. Today Nielsen is launching Twitter TV ratings, which will measure the “unique audience” of tweets about television. Nielsen is hoping to get a more complete vision of “social TV,” or measuring how many people use Twitter while watching TV. The answers can still be somewhat surprising—9.3 million people watched Grey’s Anatomy on Thursday, but just 98,600 tweeted about it. Compare that with the Breaking Bad finale, which drew 10 million viewers, but 1.2 million tweets. Some shows have been trying to increase Twitter chatter over the years—and whether increased tweets can draw in more viewers has long been a mystery.