Lawmakers face execs from Facebook, Twitter and an empty chair for Google over foreign influence operations Facebook and Twitter have tried to fight back against "inauthentic" accounts.

The committee members wanted Page to testify as well, but the company insisted on sending a lower-level official who had previously testified before the committee, irking lawmakers enough that they set up the chair to Sandberg’s right to highlight Page's absence.

In their opening remarks, committee chairman Sen. Richard Burr, a Republican from North Carolina, and vice chairman Mark Warner, a Democrat from Virginia, both expressed their “disappointment” that Google declined to send what they called an appropriately senior level official.

“Given its size and influence, I would have thought the leadership at Google would want to demonstrate how seriously it takes these challenges and to lead this important public discussion,” Warner said in prepared remarks.

In a blog post Tuesday, Google's senior vice president for global affairs and chief legal officer Kent Walker, the official sent by Google to testify in Page's place, said he will be submitting written testimony to the committee and will nevertheless be in Washington to brief lawmakers on Google’s efforts to combat possible foreign influence operations.

"We believe that we have a responsibility to prevent the misuse of our platforms and we take that very seriously," wrote Walker.

The hearing comes after Dan Coats, the Director of National Intelligence, said last month that Russia has continued to attempt the kinds of online influence operations it allegedly ran on the social platforms to spread false information and sow divisiveness ahead of the 2016 U.S. presidential election.

Throughout the hearing, Sandberg and Dorsey defended their platforms and the steps they have taken to fight foreign influence, but acknowledged past and present shortcomings.

In prepared testimony released by Facebook on Tuesday, Sandberg said that Facebook was "too slow to spot" Russia’s purported influence operation and "too slow to act."

"That’s on us," she wrote. "This interference was completely unacceptable. It violated the values of our company and of the country we love."

Facebook, she said, is "investing heavily in people and technology to keep our community safe and keep our service secure," including doubling the number of people working on safety and security issues to 20,000. Sandberg said the company is making changes to its advertisement system, letting users know who paid for what, and is working to more efficiently identify false information before users decide to share it. Among the other statistics Sandberg provided to the committee, she said Facebook disabled 1.27 billion “fake accounts” from October 2017 to March 2018.

Lawmakers also raised concerns over users' privacy, with Sen. Ron Wyden, D-Ore., calling for personal data to be considered a "national security priority."

Earlier this year Facebook founder and CEO Mark Zuckerberg testified before lawmakers in part to explain how the personal information of tens of millions of Facebook users ended up in the hands of the U.K.-based data firm Cambridge Analytica. Wednesday Sandberg spoke more broadly about privacy concerns and said that users should have control over the information they elect to share with the company.

Twitter's Dorsey tweeted out his opening statement this morning just as he gave it to the committee. He said in part, "We‘re extremely proud of helping to increase the accessibility and velocity of a simple, free and open exchange. We believe people will learn faster by being exposed to a wide range of opinions and ideas, and it helps to make our nation, and the world, feel a little bit smaller. 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."

Earlier this year the company said it was taking aggressive measures to combat inauthentic accounts.

During his testimony Wednesday, Dorsey said that Twitter is using machine learning to identify patterns of behavior that could identify automated accounts and take actions against them, including potentially identifying them as non-human. That task is made more difficult, he said, when automated accounts are programmed to behave like ones operated by humans.

In an exchange with Maine Republican Sen. Susan Collins, Dorsey said it was "unacceptable" that the lawmaker had been targeted by foreign actors over Twitter and only learned about it from researchers at Clemson University.

"We're going to do our best to make sure we catch everything and inform people when it affects them, but we are not going to catch everything," Dorsey said, emphasizing Twitter's intent to work more closely with private, industry and government partners.

Dorsey followed the Senate hearing with another hearing that could be politically more contentious -- an appearance before the House Energy and Commerce Committee that will specifically address Twitter’s "algorithms and content moderation." Twitter has been accused by some of suppressing conservative voices on the platform, which the company denied in July.

Shortly after the Senate Intelligence Committee hearing, the Department of Justice released a statement saying it planned to convene a meeting "with a number of state attorneys general this month to discuss a growing concern that these [social media] companies may be hurting competition and intentionally stifling the free exchange of ideas on their platforms."

In prepared remarks released on Tuesday by the Energy and Commerce Committee, Dorsey said his company "does not use political ideology to make any decisions, whether related to ranking content on our service or how we enforce our rules."

"We believe strongly in being impartial, and we strive to enforce our rules impartially," he said.

ABC News’ Ali Rogen, Mike Levine, Matthew Mosk and Taylor Dunn contributed to this report.