File - This Jan. 17, 2017, file photo shows a Facebook logo being displayed in a start-up companies gathering at Paris' Station F, in Paris. Facebook is on the offensive to try to contain swirling concerns about how it protects the data of its 2.2 billion members. As CEO Mark Zuckerberg prepares to face Congress on Tuesday, April 10, 2018, and the company rolls out new privacy rules, the social media juggernaut is facing the most serious challenge in its 14-year-history and seeking to maintain people’s trust and avoid a user exodus.(AP Photo/Thibault Camus, File)

NEW YORK - Facebook CEO Mark Zuckerberg prepares to face Congress on Tuesday, and No. 2 executive Sheryl Sandberg is on a media blitz as the company tries to contain swirling concerns about how it protects the data of its 2.2 billion members.

Facing its most serious challenge in its 14-year-history, the social media juggernaut is seeking to maintain people's trust and avoid a user exodus.

Here's a look at the scandal and what it means.

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HOW DID THIS START?

In mid-March, news broke that Cambridge Analytica, a London-based data-mining firm with ties to U.S. President Donald Trump, lifted the Facebook profiles of tens of millions of users without their permission in an attempt to sway elections.

A former Cambridge Analytica employee, Christopher Wylie, said the firm got Facebook information through an app in order to build psychological profiles on a large portion of the U.S. electorate. That app vacuumed up information from users who gave the app permission to access their accounts — as well as additional data from their Facebook friends. About 270,000 people were paid to take part; tens of millions of their friends were also caught in the data dragnet.

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WHY ARE PEOPLE MAD AT FACEBOOK?

The crisis appears to have crystallized the anxiety many people feel about Facebook's enormous sway over daily life and its massive data trove.

It comes on the heels of a revelation that Russia used the service to meddle in U.S. elections. It has also deepened concerns about the social media network's ability to avoid being exploited to spread propaganda and sway elections.

And it has renewed questions about Facebook's ability to protect the privacy of its users while also exploiting their personal details to fuel its lucrative advertising business.

Facebook's stock has fallen about 14 per cent since the Cambridge Analytica story broke, more than the rest of the stock market has fallen . Some advertisers and users have left Facebook, although the company has said it hasn't been a significant number yet.

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WHAT'S THE SCOPE OF THIS SCANDAL?

The number of people affected seems to keep ballooning. On Wednesday, Facebook said that as many as 87 million people might have had their data accessed — up from the 50 million disclosed in previously published reports.

And Facebook said it believes most of its 2.2 billion users have had their public profile information scraped by "malicious actors" at some point. That's through a feature that lets people search for users through their email address or phone number. While this helped individuals find friends, Facebook says malicious actors have also used this technique. Outside experts believe they could have been identity thieves, scam artists or shady data brokers assembling marketing profiles.

On top of that, some users who logged in to Facebook through Android devices discovered that Facebook had been collecting phone call logs and text histories.

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HOW HAS FACEBOOK RESPONDED?

Zuckerberg and Sandberg were silent for five days following the initial report. Zuckerberg also was criticized for initially failing to apologize when he did break the silence.

Since then, they've been on a major offensive, apologizing on national TV and rolling out new privacy policies. In a call with reporters Wednesday, Zuckerberg acknowledged he made a "huge mistake" in failing to take a broad enough view of what Facebook's responsibility is in the world.

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IS FACEBOOK CHANGING ITS PRACTICES?

Facebook has outlined a new privacy policy that aims to explain the data it gathers on users more clearly. It's also restricting access that apps can have to data about users' events, as well as information about groups such as member lists and content. In addition, the company is removing the option to search for users by entering a phone number or an email address.

Facebook is trying to be more upfront with users. On Monday, all Facebook users will receive a notice on their Facebook feeds with a link to see what apps they use and what information they have shared with those apps. They'll have a chance to delete apps they no longer want. Users who might have had their data shared with Cambridge Analytica will be told of that.

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WHY IS WASHINGTON INVOLVED?

Congress has been interested in increasing regulation on Facebook due to its size and the myriad of issues it's facing. That includes data privacy, fake news and Russian meddling in the U.S. elections.

Zuckerberg is set to testify on the privacy scandal Tuesday before a joint hearing of the Senate Commerce and Judiciary Committees, and a day later before the House Energy and Commerce Committee. The two sessions will be his first testimony before Congress. Separately, the U.S. Federal Trade Commission and various authorities in Europe are investigating.

Meanwhile, special counsel Robert Mueller is scrutinizing the connections between Trump's campaign and Cambridge Analytica so he'll be interested in whatever Zuckerberg has to say, too.

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AP Business Writers Alexandra Olson and Barbara Ortutay in New York contributed to this report.

News from © The Associated Press, 2018