What happens to the data you post on Facebook? And who's responsible for how those personal details are used?
Facebook is under intense pressure to answer these questions - and more - after it admitted that a company linked to President Donald Trump's campaign had accessed and improperly stored a huge trove of its user data.
The controversy erupted as UK media and The New York Times reported that data analysis firm Cambridge Analytica tried to influence how Americans voted using information gleaned from millions of Facebook profiles.
Here's what you need to know.
Facebook said it gave permission to University of Cambridge psychology professor Aleksandr Kogan to harvest information from users who downloaded his app - "thisisyourdigitallife."
The app offered a personality test. But Facebook users who downloaded the app also gave the professor permission to collect data on their location, their friends and content they had "liked."
That was allowed under Facebook's rules at the time.
The New York Times, however, reported that Kogan provided that data - which included information from over 50 million profiles - to Cambridge Analytica, breaching Facebook's rules.
Cambridge Analytica was working to develop techniques that could be used to influence voters.
Facebook said it asked Cambridge Analytica to delete the data in 2015, but learned several days ago from "reports" that not all of it had been purged.
Cambridge Analytica said that the data set revealed by The New York Times was not used "as part of the services it provided to the Donald Trump 2016 presidential campaign."
The reports in The Times and other media were based in part on interviews with former Cambridge Analytica contractor and self-styled whistle-blower Christopher Wylie.
Why does this matter?
Facebook has been unable to shake off questions over its role in the 2016 presidential election.
CEO Mark Zuckerberg initially expressed skepticism that Facebook could have been used to influence voters, but a series of revelations over Russian meddling have caused the company to make big changes in recent months.
It has sought to crack down on fake news, undermine the business model used by trolls and make political advertising more transparent.
Zuckerberg now has a whole new set of questions to address: Was Facebook transparent enough with users about how their information would be used? Should it have done more to keep tabs on how third parties were using data?
There could be major implications for the company's business model, which is based on selling user data to app developers and advertisers.
Lawmakers and regulators have already seized on the controversy.
Massachusetts Attorney General Maura Healey said Saturday that her office is opening an investigation into Facebook and Cambridge Analytica. The UK Information Commissioner's Office is also investigating, as is the European Union parliament.
"This is a big deal. ... The privacy violations there are significant," Republican Senator Jeff Flake told CNN. "The question is, who knew it? When did they know it? How long did this go on?"
What happens next?
Lawmakers have called on Zuckerberg to explain his company's actions.
Amy Klobuchar, a Democrat who serves on the Senate Judiciary Committee, said on Saturday that "Zuckerberg needs to testify."
"This is a major breach that must be investigated," she said on Twitter. "It's clear these platforms can't police themselves."
The revelations are likely to fuel calls for more regulation of tech companies. The industry is already scrambling to prepare for tough new data privacy rules in Europe, and similar measures could be considered elsewhere.
Facebook has promised to conduct a "comprehensive internal and external review." For now, it can't say for sure what happened to your data.