Although people are watching their shows, the networks are loath to release data about how many people are watching TV shows online and how often. The reason? Possibly because Internet viewers are worth only a fraction of the advertising dollars of television viewers.

“The four and a half billion we make on broadcast is never going to equate to four and a half billion online,” said Quincy Smith, the president of CBS Interactive.

The most popular television shows tend to be the most-viewed online as well. While the doctors and nurses of the hit ABC drama Grey’s Anatomy look a little pixelated on a computer monitor, episodes of the show have been streamed more than 26 million times on ABC.com in the last six months, adding the equivalent of two full ratings points to each telecast. Heroes, Ugly Betty, CSI, House and Gossip Girl are among the other online hits, analysts say.

Just how many shows are being streamed is unclear because there is no widely recognised version of the Nielsen TV ratings for the Internet yet. Regardless of the content, the shift is forcing the networks to rethink the long-held axioms of network schedulers and advertisers.

In an address in January to television executives in Las Vegas, Jeff Zucker, the chief executive of NBC Universal, noted that NBC.com had measured more than half a billion video streams in just over a year. “Our challenge with all these ventures is to effectively monetize them so that we do not end up trading analog dollars for digital pennies,” Zucker said, calling it the No 1 challenge for the industry.