BBC: Cambridge Analytica The Story so far
It's a sensational story containing allegations of sleaze, psychological manipulation and data misuse that has provoked an internationally furious response.
Tech giant Facebook and data analytics firm Cambridge Analytica are at the centre of a dispute over the harvesting and use of personal data - and whether it was used to influence the outcome of the US 2016 presidential election or the UK Brexit referendum.
Both firms deny any wrongdoing.
How has Cambridge Analytica been accused of sleazy tactics?
Channel 4 News sent an undercover reporter to meet executives from data analytics firm Cambridge Analytica.
The firm had been credited with helping Donald Trump to presidential victory.
The reporter posed as a Sri Lankan businessman wanting to influence a local election.
Cambridge Analytica boss Alexander Nix was apparently filmed giving examples of how his firm could discredit political rivals by arranging various smear campaigns, including setting up encounters with prostitutes and staging situations in which apparent bribery could be caught on camera.
The firm denies all the claims and says the documentary was "edited and scripted to grossly represent the nature of those conversations". It claims the conversations were led by the reporters.
"I must emphatically state that Cambridge Analytica does not condone or engage in entrapment, bribes or so-called 'honeytraps', and nor does it use untrue material for any purpose," said Mr Nix.
What was Facebook's role?
In 2014 a quiz on Facebook invited users to find out their personality type.
It was developed by University of Cambridge academic Aleksandr Kogan (the university has no connections with Cambridge Analytica).
As was common with apps and games at that time, it was designed to harvest not only the user data of the person taking part in the quiz, but also the data of their friends.
Facebook has since changed the amount of data developers can scrape in this way.
Christopher Wylie, who worked with Cambridge Analytica, alleges that because 270,000 people took the quiz, the data of some 50 million users, mainly in the US, was harvested without their explicit consent via their friend networks.
Mr Wylie claims the data was sold to Cambridge Analytica, which then used it to psychologically profile people and deliver pro-Trump material to them.
Cambridge Analytica denies any of it was used as part of the services it provided to the Trump campaign.
Is this against Facebook's terms?
The data was gathered using Facebook's infrastructure at that time, and many other developers had taken advantage of it - but the data was not authorised for them to share with others.
The other key point is that even the people directly taking part in the personality quiz would have had no idea that they were potentially sharing their data with Donald Trump's election campaign.
Facebook say when they learned their rules had been breached, they removed the app and demanded assurances that the information had been deleted.
Cambridge Analytica claims that it never used the data, and deleted it when Facebook told it to.
Both Facebook and the UK Information Commissioner want to find out whether it was properly destroyed, as Mr Wylie claims it was not.
What has the official response been?
US senators have called on Mark Zuckerberg to testify before Congress about how Facebook will protect users.
The head of the European Parliament said it would investigate to see if the data was misused.
A spokesman for Prime Minister Theresa May said she was "very concerned" about the revelations.
How can you protect your data?
There are a few things to be aware of if you want to restrict who has access to your data.
- Keep an eye on apps, especially those which require you to log in using your Facebook account - they often have a very wide range of permissions and many are specifically designed to pick up your data
- Use an ad blocker to limit advertising
- Look at your Facebook security settings and make sure you are aware of what is enabled. Check the individual app settings to see whether you have given them permission to view your friends as well as yourself.
- You can download a copy of the data Facebook holds on you, although it is not comprehensive. There is a download button at the bottom of the General Account Settings tab. However bear in mind that your data may be less secure sitting on your laptop than it is on Facebook's servers, if your device is hacked.
You can of course, simply leave Facebook, but the campaign group Privacy International warns that privacy concerns extend beyond the social network.
"The current focus is on protecting your data being exploited by third parties, but your data is being exploited all the time," said a spokeswoman.
"Many apps on your phone will have permission to access location data, your entire phone book and so on. It is just the tip of the iceberg."
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