Facebook chief executive Mark Zuckerberg has apologised to Britons over a "breach of trust", taking out full-page advertisements in British Sunday newspapers after a political consultancy got its hands on data on 50 million users.
"We have a responsibility to protect your information. If we can't, we don't deserve it," said the advert, signed by Facebook founder Mr Zuckerberg.
The world's largest social media network is facing growing government scrutiny in Europe and the United States.
This follows allegations by a whistleblower that British consultancy Cambridge Analytica improperly accessed users' information to build profiles on American voters that were later used to help elect US President Donald Trump in 2016.
The plain black text apology on a white background, with only a tiny Facebook logo, appeared in Sunday publications including The Observer - one of the newspapers whose reporting on the issue has sent Facebook's share price tumbling.
- India worried Cambridge Analytica might screw with its election
- Political firm Cambridge Analytica accused of rigging elections
Mr Zuckerberg said an app built by a university researcher "leaked Facebook data of millions of people in 2014".
"This was a breach of trust, and I'm sorry we didn't do more at the time," Mr Zuckerberg said, reiterating an apology first made last week in US television interviews.
Cambridge Analytica says it initially believed the data had been obtained in line with data protection laws, and later deleted it at Facebook's request. The consultancy said it did not use the data in work it did for the 2016 US election.
On Friday night, investigators from Britain's data watchdog searched the London offices of Cambridge Analytica for several hours.
Mr Zuckerberg, whose firm has lost more than US$50 billion in market value since the allegations, said Facebook would give users more information and control about who can access their data.
"Thank you for believing in this community. I promise to do better for you," he wrote.
Advertisers Mozilla and German bank Commerzbank have suspended ads on the service and the hashtag #DeleteFacebook has been trending online.