Broadcast television, newspapers and magazines.
Basically, those media outlets that are considered traditional but are being made obsolete by faster, more malleable technologies (blogs, podcasts, streaming content on mobile devices).
The phrase has gained provenance as traditional media sources shrink and lay off staff -- a slow extinction that mirrors the disappearance of dinosaurs, though with fewer explosions.
"I just got a job as a reporter at the Denver Post!" screamed Danny.
"Ugh," groaned Danny's girlfriend. "Why would you want to work for dinosaur media?"