UPDATE: Tuesday's Senate hearing ended at 7:25pm ET.
Some key Russia-related takeaways:
CNN:
When asked by Democratic Sen. Patrick Leahy while he testified on Capitol Hill whether Facebook had been served subpoenas for the special counsel, Zuckerberg responded "yes," but later clarified: "I am actually not aware of a subpoena. I'm aware that there may be, but we are working with them."
When asked if his employees had been interviewed, he also responded yes, but added, "I have not."
He continued: "I want to be careful here because our work with the special counsel is confidential and I want to make sure that in an open session I'm not revealing something that is confidential."
NYT:
Senator Dianne Feinstein, the top Democrat on the Judiciary Committee, pressed Mr. Zuckerberg on Russia’s exploitation of the platform during the 2016 presidential election.
Mr. Zuckerberg admitted that the company’s effort to find and stop the Russian meddling was “slow,” and called that failure “one of my greatest regrets.” He said Facebook was tracking known Russian hacking groups in real time but took much longer to recognize the inflammatory posts of the Internet Research Agency, a private company with Kremlin ties.
“There are people in Russia whose job is to exploit our systems,” Mr. Zuckerberg said. “This is an arms race.”
But the Facebook founder said the company deployed new artificial intelligence tools to detect malicious activity in elections in France, Italy and a special Senate race in Alabama. He said he believed the new technology would help protect the integrity of elections around the world from manipulation via Facebook.
Also, Senator Amy Klobuchar (D-MN) asked Zuckerberg if Facebook has found any overlap between the 87 million users whose data Cambridge Analytica, which worked for the Trump campaign, acquired illicitly and the 127 million users who likely saw pages and ads from the Kremlin-linked Internet Research Agency troll factory, especially given how close the 2016 election results were in certain states.
Zuckerberg said that is something Facebook is investigating now, and it is entirely possible there will be a connection there.
On Wednesday, Zuckerberg will appear before the House Energy and Commerce Committee starting at 10am ET. The committee's stream is online here.
Mark Zuckerberg Testimony: Senators Question Facebook’s Commitment to Privacy (NYT)
Zuckerberg confirms Facebook is working with special counsel Robert Mueller's investigation (CNN)
Mark Zuckerberg, Chairman and Chief Executive Officer of Facebook, testifies before the Senate Judiciary and Commerce Committees in a joint hearing titled, "Facebook, Social Media Privacy, and the Use and Abuse of Data."
Watch:
Read Senate Judiciary Committee Chairman Chuck Grassley's (R-IA) prepared statement.
Read Mark Zuckerberg's prepared statement.
Follow our live coverage on Twitter.
The Associated Press' live blog is here.