The Facebook scandal involving the harvesting of data from tens of millions of users has raised a lot of questions about social media and search engines.
As Facebook founder and CEO Mark Zuckerberg testifies before the US Congress this week on protecting user data, here is a primer on what they know about you:
Facebook, which has more than two billion users, has access to everything you do on the site: the photos and videos you post, your comments, your 'likes,' anything you share or consult, the identity of your friends and any other users you interact with, your location and other information.
Ditto for Instagram and WhatsApp, which are owned by Facebook, and for Snapchat and Twitter. A user can control some sharing of their Facebook data with privacy settings and the ad preferences page.
- What it sells: Facebook insists it does not sell advertisers personally identifiable information or even aggregate data. What it provides an advertiser with is the ability to reach a specific demographic, which enhances the effectiveness of an ad campaign. Twitter, for its part, provides access to an internal search engine that sweeps up all messages on the site.
What it shares: Most social media platforms are open to outside developers who create apps fed in varying degrees by using data from users of these networks. In the case of Facebook, the public profile -- the whole page for some people, or just the first and last name and photo for others -- does not require authorization from the user, but accessing the rest may require a separate OK from the user.
Once data is mined by outside apps, it is no longer in the grasp of Facebook and trying to get hold of it again is difficult.
"Once people had access to that data, Facebook has no way of knowing for sure what they did with that data," said Ryan Matzner, co-founder of mobile app designer Fueled. "It's like sending an email to somebody and then saying: 'What did they do with that email?' You don't know."
Only bank and payment details held by Facebook are off-limits.
- What they collect: Google, Yahoo and Bing gather all information involving searches including the websites that are accessed and the location of the user. This can be integrated with information from other services owned by the internet giants.
"You don't have to tell Google your age and your gender and all those things. They can determine all of that based on so many other factors," said Chirag Shah, a computer science professor at Rutgers University.
- What they sell: like social networks, their revenue comes largely from advertising. They do not sell data, but rather access to a consumer with very specific characteristics.
This comes from compiling search engine data but also, in the case of Google, from searches and content viewed on its YouTube platform. Google used to also mine the content of Gmail before ending this practice in June.
- What they share: Like social media networks, search engines share data with developers and third-party app makers.
Are there limits?
In the United States there are practically no laws against the use of data from social media or search engines.
But the Federal Trade Commission did sanction Facebook in 2011 for its handling of personal data.
In Canada and Europe, there are some limits on the use of data, mainly involving health.
Facebook was fined 110 million euros ($135.7 million) by the European Commission last year for sharing personal data with WhatsApp.
In an attempt to harmonise data privacy laws, the EU's General Data Protection Regulation is to go into force on May 25.