Senators raised the prospect of regulating social media platforms as Twitter’s Jack Dorsey and Facebook’s Sheryl Sandberg testified this morning about efforts to combat foreign manipulation of their platforms.

Sen. Richard Burr, the chairman of the Senate Intelligence Committee, commended Facebook and Twitter for dedicating the resources to fight corruption and misuse of their platforms, and their willingness to collaborate with government and law enforcement. But he said continued abuse of social media underscores a national security vulnerability that remains unaddressed.

“If the answer is regulation, let’s have a dialog about what that looks like,” Burr said in his opening remarks. “If it requires information sharing and government cooperation, let’s get it out there.”

Dorsey, who flouted stodgy Washington, D.C., protocol, by appearing in a suit without a tie and sporting a nose ring, admitted he is “typically pretty shy,” and read a statement off his cell phone while simultaneously tweeting his testimony.

“We aren’t proud of how that free and open exchange has been weaponized and used to distract and divide people, and our nation. We found ourselves unprepared and ill-equipped for the immensity of the problems we’ve acknowledged,” Dorsey said. “Abuse, harassment, troll armies, propaganda through bots and human coordination, misinformation campaigns, and divisive filter bubbles…that‘s not a healthy public square. Worse, a relatively small number of bad-faith actors were able to game Twitter to have an outsized impact.”

Sandberg, who worked as deputy Treasury Secretary in the Clinton administration, appeared more at ease on Capitol Hill, and stuck closely to her prepared remarks that underscored Facebook’s efforts to shut down fake accounts, limit the distribution of posts identified as false and increasing the transparency in political advertising.

“Senators, let me be clear. We are more determined than our opponents and we will keep acting,” Sandberg said. “When bad actors try to use our site we will block them. When opponents use new techniques, we will share them. We need to be ever more vigilant. As Chairman Burr noted, nothing less than our democratic institutions are at stake.”

Google, whose absence was underscored by an empty chair placed in the hearing room, came in for harsh criticism for refusing to send Larry Page, chief executive of Google’s parent, Alphabet, or Google CEO Sundar Pichai.

“I’m disappointed that Google decided against sending the right senior level executive,” said Burr, whose committee rejected the company’s offer of testimony from Chief Legal Officer Kent Walker.

Republican Sen. Tom Cotton speculated about the reasons Google’s top executives choose not to participate in the hearing, saying they probably hoped to avoid answering questions about reports that the company is developing a search engine that would comply with Chinese censorship rules or its decision not to renew a contract with the Pentagon for artificial intelligence work.

“The silence we hear right now from the Google chair would be reminiscent of the silence we’d hear from Google,” Cotton said.

Committee Vice Chairman Sen. Mark Warner, a Democrat from Virginia, said he has unanswered questions about Google’s continued problems in surfacing conspiracy theories in search results, or the “divisive videos” found on YouTube, or attempts to hack gmail.

“Google has immense responsibility in this space, given its size and influence,” Warner said. “I would have thought that leadership at Google would like to demonstrate how seriously it takes these issues. Unfortunately it did not choose to make that decision.”

Sandberg and Dorsey highlighted efforts to crack down on abuse of their platforms.

Facebook said its commitment of resources — doubling the number of people working on safety and security and improving machine learning technology — have allowed it to proactively identify abuse, and remove or add warning labels often before the problem is reported by users.

Twitter says it’s removing more 200% more accounts for violating its policies, challenging 8 to 10 million suspicious accounts every week and thwarting more than half a million accounts from logging in to Twitter every day. Dorsey said the company is committing the people and resources to create a “healthy” public square.

“When I think of our work, I think of my mom and dad in St. Louis, a Democrat and a Republican,” Dorsey said. “For them Twitter has always been a source of joy, learning and connection to something bigger. They’re proud of me, proud of Twitter, and proud of what made it all possible.”

Sen. Angus King, an Independent from Maine, and Sen. James Lankford, a Republican from Oklahoma, both expressed concern about the next wave of digital deception, something called “deep fake,” realistic-looking bogus audio and video created with the aid of artificial intelligence.

Lankford worried aloud about what will happen when American’s can’t trust what they see with their own eyes.

“That is a very different day for video sharing,” he said, urging Facebook and Twitter to purse these fake videos with the same vigor as they’ve tackled child pornography. “You’ll aggressively go after these things.”

Sen. Ron Wyden, a Democrat from Oregon, pressed Facebook about its arrangements with phone makers that allow devices to access data from the social network’s users, a matter than the Washington Post said could subject the social network to additional scrutiny by the Federal Trade Commission.

Wyden called on Sandberg to make public what information was provided to smartphone manufacturers, including China’s Huawei and ZTE.

“The American people deserve to see this information,” Wyden said.

Sandberg said Facebook would “work with you about what information we can share.”

Maine Republican Sen. Susan Collins had a more personal concern, saying she learned from researchers at Clemson University that she had been targeted by Russia’s disinformation campaign, by 261 tweets that reached some 360,000 followers.

“It seems you should notify people,” Collins said.

“I agree,” Dorsey responded. “It’s unacceptable. We want to find ways to work more openly, not just with our peer companies, but with researchers in university and with law enforcement. They all bring a different perspective to our work.”