Facebook CEO Mark Zuckerberg disclosed Tuesday his company is "working with" special counsel Robert Mueller in the federal probe of Russian interference in the 2016 presidential campaign - and working hard to change its own operations after the harvesting of users' private information by a Trump campaign-affiliated data-mining company.

The founder of the social media giant publicly apologised for his company's errors in failing to better protect the personal information of its millions of users, a controversy that has brought a flood of bad publicity and sent the company's  stock value plunging.

He seemed to achieve a measure of success: Facebook shares surged 4.5 percent for the day, the biggest gain in two years.

Zuckerberg told the Senate Judiciary and Commerce committees that he has not been personally interviewed by Mueller's team, but "I know we're working with them."

He offered no details, citing a concern about confidentiality rules of the investigation.

Earlier this year Mueller charged 13 Russian individuals and three Russian companies in a plot to interfere in the 2016 presidential election through a social media propaganda effort that included online ad purchases using US aliases and politicking on US soil. A number of the Russian ads were on Facebook.

During Tuesday's at-times-contentious hearing, Zuckerberg said it had been "clearly a mistake" to believe the data-mining company Cambridge Analytica had deleted user data that it had harvested in an attempt to sway elections. He said Facebook had considered the data collection "a closed case" because it thought the information had been discarded.

Facebook also didn't alert the Federal Trade Commission, Zuckerberg said, and he assured senators the company would handle the situation differently today.

He began a two-day congressional inquisition with a public apology for the way Facebook handled the data-mining of its users' data. He took responsibility for failing to prevent Cambridge Analytica, which was affiliated with Donald Trump's presidential campaign, from gathering personal information from 87 million users.

"I think he went a long ways toward giving the kind of assurance that Facebook certainly wants to come out of this kind of hearing. I don't think he was nicked too badly," said David Levine, a professor at the University of California Hastings College of the Law.

Separately, the company began alerting some of its users that their data was gathered by Cambridge Analytica. A notification that appeared on Facebook for some users Tuesday told them that "one of your friends" used Facebook to log into a now-banned personality quiz app called "This Is Your Digital Life." The notice says the app misused the information, including public profiles, page likes, birthdays and current cities, by sharing it with Cambridge Analytica.

Berkeley, California – 10 April 2018

1. David Levine, law professor at University of California Hastings College of the Law, watching Facebook CEO Mark Zuckerberg testimony in living room at home

2. Levine sitting in chair, watching testimony

3. TV screen showing Zuckerberg testimony

4. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"Mark Zuckerberg had a tall order today. He really had a lot to get done, which is he had to assure members of the Senate that Facebook was on top of the problem, that they had apologised, that they understood the scope of the problems, and that Facebook was taking significant measures to correct those problems. Mr. Zuckerberg also needed to reassure the billions of users around the world that the safety and security of users' data is paramount in the minds of the top officials at Facebook and that they want to make sure that it's as protected as possible."

5. Levine watching Zuckerberg testimony in living room

6. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"He was very good at looking sincere the way he needs to, looking contrite, making eye contact when he needed to, looking down when he needed to look a little 'I'm hearing you' that sort of thing, so he certainly looked coached. He looked sincere. He looked like he wanted to work with the senators. But he also held his position. If there were issues about will you support a certain type of legislation, where obviously Facebook doesn't really want to support it, he said 'well I agree with the general principle and certainly our team can work with your people,' but when they tried to corner him about 'will you support certain legislation' he wouldn't do it."

7. Zuckerberg and hearing on television

8. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"I think he went a long ways toward giving the kind of assurance that Facebook certainly wants to come out of this kind of hearing. I don't think he was nicked too badly. I think there were a couple questions around Cambridge Analytica where he sounded more ignorant, didn't have facts on his fingertips in the way that maybe he should have. But by and large I think at Facebook headquarters they're going to say, certainly at the end of today, there are still more hearings, but - mission accomplished."

9. Levine watching Zuckerberg testimony in living room

Facebook CEO Mark Zuckerberg disclosed Tuesday his company is "working with" special counsel Robert Mueller in the federal probe of Russian interference in the 2016 presidential campaign - and working hard to change its own operations after the harvesting of users' private information by a Trump campaign-affiliated data-mining company.

The founder of the social media giant publicly apologized for his company's errors in failing to better protect the personal information of its millions of users, a controversy that has brought a flood of bad publicity and sent the company's  stock value plunging. He seemed to achieve a measure of success: Facebook shares surged 4.5 percent for the day, the biggest gain in two years.

Zuckerberg told the Senate Judiciary and Commerce committees that he has not been personally interviewed by Mueller's team, but "I know we're working with them." He offered no details, citing a concern about confidentiality rules of the investigation.

Earlier this year Mueller charged 13 Russian individuals and three Russian companies in a plot to interfere in the 2016 presidential election through a social media propaganda effort that included online ad purchases using U.S. aliases and politicking on U.S. soil. A number of the Russian ads were on Facebook.

During Tuesday's at-times-contentious hearing, Zuckerberg said it had been "clearly a mistake" to believe the data-mining company Cambridge Analytica had deleted user data that it had harvested in an attempt to sway elections. He said Facebook had considered the data collection "a closed case" because it thought the information had been discarded.

Facebook also didn't alert the Federal Trade Commission, Zuckerberg said, and he assured senators the company would handle the situation differently today.

He began a two-day congressional inquisition with a public apology for the way Facebook handled the data-mining of its users' data. He took responsibility for failing to prevent Cambridge Analytica, which was affiliated with Donald Trump's presidential campaign, from gathering personal information from 87 million users.

"I think he went a long ways toward giving the kind of assurance that Facebook certainly wants to come out of this kind of hearing. I don't think he was nicked too badly," said David Levine, a professor at the University of California Hastings College of the Law.

Separately, the company began alerting some of its users that their data was gathered by Cambridge Analytica. A notification that appeared on Facebook for some users Tuesday told them that "one of your friends" used Facebook to log into a now-banned personality quiz app called "This Is Your Digital Life." The notice says the app misused the information, including public profiles, page likes, birthdays and current cities, by sharing it with Cambridge Analytica.

Berkeley, California - April 10, 2018

1. David Levine watches Mark Zuckerberg testimony in living room at home

2. Levine sitting in chair, watching testimony

3. TV screen showing Zuckerberg testimony

4. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"Mark Zuckerberg had a tall order today. He really had a lot to get done, which is he had to assure members of the Senate that Facebook was on top of the problem, that they had apologized, that they understood the scope of the problems, and that Facebook was taking significant measures to correct those problems. Mr. Zuckerberg also needed to reassure the millions of users around the world that the safety and security of users' data is paramount in the minds of the top officials at Facebook and that they want to make sure that it's as protected as possible."

5. Levine watching Zuckerberg testimony in living room

6. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"He was very good at looking sincere the way he needs to, looking contrite, making eye contact when he needed to, looking down when he needed to look a little - 'I'm hearing you' that sort of thing, so he certainly looked coached. He looked sincere. He looked like he wanted to work with the senators. But he also held his position. If there were issues about will you support a certain type of legislation, where obviously Facebook doesn't really want to support it, he said 'well I agree with the general principle and certainly our team can work with your people,' but when they tried to corner him about ' will you support certain legislation' he wouldn't do it."

7. Zuckerberg on television

8. SOUNDBITE (English) David Levine, law professor, University of California Hastings College of the Law:

"I think he went a long ways toward giving the kind of assurance that Facebook certainly wants to come out of this kind of hearing. I don't think he was nicked too badly. I think there were a couple questions around Cambridge Analytica where he sounded more ignorant, didn't have facts on his fingertips in the way that maybe he should have. But by and large I think at Facebook headquarters they're going to say, certainly at the end of today there still more hearings, but - mission accomplished."

9. Levine watching Zuckerberg testimony in living room