Zalaznick, Tellem and Sweeney (l. to r.)

"If Twitter is the telephone, we're the conversation."

That comment was a highlight of Fortune Brainstorm Tech's "Future of Television" discussion on Thursday—even though the person who said it wasn't in the room.

No offense to the panelists on stage: Disney (DIS) Media Networks co-chair Anne Sweeney, CBS (CBS) exec Nancy Tellem, and NBC Universal's (CMCSA) Lauren Zalaznick. But the remark--which moderator Jason Hirschhorn said former MTV Networks (VIA) CEO Judy McGrath uttered to him--nails the point about what programmers must do today: Go two-way and way beyond the TV screen.

What makes a hit? TV ratings still matter, of course, but social engagement can make or break a show in this digital era. Tellem, who is senior advisor to CBS chief Leslie Moonves,  pointed to Gossip Girl on the CW network (of which CBS is a part owner). The series, by measure of TV eyeballs when episodes premiere, might be considered a dud--or at least "a conundrum," which is what Tellem called the show. But TV execs learned to love Gossip Girl because the target--young women and teenage girls--are passionate viewers online, via iTunes (AAPL), Hulu, Netflix (NFLX), video on demand, DVR, whatever. Plus, Gossip Girl's 9.8 million Facebook fans chat and tweet up a storm.

Meanwhile, on ABC, Grey's Anatomy and Modern Family have huge audiences who time-shift viewing via DVR--and typically skip commercials. Which is why Zalaznick says: "The DVR is the biggest legal pirate there ever was....We've enabled legal piracy." Zalaznick, who oversees NBCU's Entertainment & Digital Networks and Integrated Media, made the point that TV execs must learn how to make good money serving consumers who want their TV shows anytime and everywhere.

Moreover, building a hit is ever more complicated. Big social buzz—top Twitter trends, for instance--is less predictive of big TV ratings than it used to be, said Zalaznick. The reason? Social-media noise is getting so loud, it's challenging forecasters. "We're at a turning point," she added.

So the TV execs soldier on. The web's exploding social-media platforms--Twitter, Facebook, and other second and third "screens" beyond the tube—invite them to get a lot more creative with their products and build profits too. Meanwhile, Zalaznick reminded the group of the everlasting No. 1 challenge: "It is really hard to make excellent stuff about compelling characters that you want to see over and over again."