CBS Corp. 's "The Big Bang Theory" and "NCIS" are among the most-watched shows on television, with upward of 20 million viewers each. But in Nielsen's new ranking of programs with the greatest reach on Twitter, they aren't even in the top 10.

The Nielsen data, which will be published for the first time Monday, provides new details on the number of tweets about TV shows and the size of the audience that sees them. It highlights that the programs that generate the most Twitter activity often aren't the ones with the highest viewership.

For TV networks and advertisers looking to exploit the buzz about TV shows on Twitter, the data are a reminder that the social-media service's user base has a very different makeup than the mass-market TV-viewing audience that marketers spend tens of billions of dollars each year to reach. Twitter's 49.2 million U.S. users generally skew younger and are disproportionately in cities, for example, according to marketers and media analysts.

Still, marketers see potential value in knowing which shows have Twitter enthusiasts. For some, it is becoming a factor when they purchase TV time: The theory is that if fans are engaged on Twitter that "means the ads are also being paid attention to," said Steve Kalb, director of video investment at Mullen, an advertising firm owned by Interpublic Group of Cos. Mr. Kalb said the approach is still in the "experimental" phase.

The stakes are high for Twitter, which is under the spotlight of an initial public offering of stock and has identified TV-related ad spending as a crucial revenue driver. The company has joined with several TV networks through its Amplify program, in which short video clips are distributed on Twitter with ads embedded. Another service allows marketers to target Twitter users who have seen one of their ads on TV.