Lawmakers worry about terrorists on Twitter

 "al-Qaida was the first to fully exploit social networks," Jenksins said. "It's a global movement that therefore requires a global network to support it."

But the experts told the panel that people are unlikely to join terrorist groups just by viewing radical material on social media sites.

"By itself, the Internet doesn't get them all the way there," Jenkins said, arguing that people usually need face-to-face peer pressure before they will commit acts of violence.

When Rep. Jackie Speier (D-Calif.) asked whether the use of social media by terrorists should keep her up at night, William McCants, an analyst at the Center for Naval Analyses, said, "No." 

He said there is no evidence that large numbers of people are swayed by terrorist propaganda online. 

Andrew Aaron Weisburd, director of the Society for Internet Research, said Internet providers and social media sites should be more pro-active in blocking violent material. 

"While it is in no one's interests to prosecute Internet service providers, they must be made to realize that they can neither turn a blind eye to the use of their services by terrorist organizations, nor can they continue to put the onus of identifying and removing terrorist media on private citizens," he said. "I don't believe that Google, operator of YouTube, has an interest in promoting violent extremism, and they have already made some effort to address this issue, but they can and should do more."

The experts noted that social media can also provide the U.S. government with a valuable intelligence tool to monitor extremist groups.

But McCants noted that social networks such as Facebook that allow users to make their information private are more challenging to use for intelligence-gathering.

"You can't just make a friend request and expect it to be answered," he said.