The biggest challenge to Cable News Network is not other 24-hour television news stations but social networks like Facebook and Twitter, the president of CNN US said Wednesday.

"The competition I'm really afraid of are social networking sites," Jon Klein said at the Bloomberg BusinessWeek 2010 Media Summit here. "That's an alternative that threatens to pull people away from us.

"The people you're friends with on Facebook or the people you follow on Twitter are trusted sources of information," Klein said. "You click on links they send to you and you trust them.

"Well, we want to be the most trusted name in news," he said. "We don't want the 1,000 people you follow in Twitter to be the most trusted sources for you.

"That's a challenge and we have to rise to that challenge," Klein said.

"So I'm far more worried about the 500 million people on Facebook than I am about two million people watching Fox," the News Corp-owned station which is CNN's major competitor in the cable news arena, Klein said.

The CNN executive said his network's "mission" is to drive social network and other Web users to "link back to something on CNN."

Besides expanding its footprint on the Internet with news and video, CNN is looking at mobile devices, Klein said.

"Online is a big growth area for us, mobile has enormous growth potential and domestic US cable is actually a growth area," he said. "There's a lot of room to grow.