The backlash against Facebook has had some people hunting for the "delete" button on the social network, but even privacy conscious internet users most don't realise that even if you've tried to stay a stranger to Facebook the social network still knows a whole lot about you.

Yesterday in a hearing in the US Congress, Facebook chief executive Mark Zuckerberg was pressed over the company's tracking of users outside of Facebook. Politicians questioned how Facebook gathers data from non-users, so-called "shadow profiles", or data on people outside the social network.

Zuckerberg has repeatedly said that Facebook users alone have control over what data they choose to share with Facebook. But US representative Ben Lujan of New Mexico, berated Zuckerberg over the lack of transparency over third-party tracking.

"You are collecting data on people who are not even Facebook users", Lujan said. "Who have never signed a privacy agreement."

Lujan said that even when users who don't have a Facebook account who want to find their data the site may hold on them, Facebook redirects them to a page where they have to log-in to their, non-existent, account to find out what Facebook knows.

Here is some of the data Facebook can scrape from you, even if you don't have a profile:

Name, email and phone number from your friends

When a user signs up for Facebook, the site prompts them to upload a list of contacts so Facebook can try and find friends for you. There is also a tab in the Facebook app for uploading contacts from your smartphone.

When starting up the app, one of the first things Facebook suggests is syncing your contacts with the app - including data on names, email and phone numbers.

While this may seem innocuous, as it allows Facebook to send out invites to other friends, importantly it means it has already started building up the first pieces of profile data about people who have never consented to have their data shared.

Setting up a new Facebook app account, you are immediately asked for contact access

Facebook also uses this data to build up its People You May Know service, which tries to link you to people it thinks are friends. Again, this comes under Facebook's "mission" of connecting people, but not everybody will want that personal data to be accessed by the social network.

And, when you go to Facebook to try and check if it has collected this data, the site directs you to your Facebook profile, even if you have never created one in the first place.

Data from Like and Share buttons on other websites

There is another way that Facebook can pick up information about you, even if you don't sign up to the site. Have you seen those little Like and Share buttons on most websites? Those are Facebook buttons, installed on websites to make sharing of articles and links easier.

Of course, these buttons are hosted by Facebook itself. So if you visit a page with one of these buttons, the likelihood is Facebook can see data about where you have been browsing and what other Facebook linked sites you have visited. It won't give up any personally identifiable information, but it is your browsing data that Facebook is able to see.

Facebook Like buttons can be used to track on the web credit: Reuters

Data from Facebook Pixel and web trackers

Even more hidden than these sharing tools is Facebook Pixel, a web tracker that lives in many third-party Facebook pages. The Pixel is a tool for advertisers, designed to measure all kinds of analytics, and again is in common use by the industry.

However, the Pixel makes it possible to track Facebook users even if they are logged out of Facebook by monitoring data like their IP address. This can be used, for example, to follow a user and target an advert them if they have added an item to their shopping cart in an only store, but not decided to buy it yet.

That said, lots of websites use similar tools and trackers, but for Zuckerberg to claim the user has significant control over this is not correct. Facebook has been engaged in a legal dispute against Belgium over the very use of these trackers, and it remains to be seen if GDPR will require changes in how this works.

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It can't be denied that Facebook is an advertising giant and that data is its currency (despite one senator appearing confused and asking how Facebook makes money without subscriptions), but it will worry some not on Facebook that the site can build up a profile of them without their knowledge.

As Rep. Lujan put it: "You’ve said everyone controls their data, but you’re collecting data on people who are not even Facebook users who have never signed a consent, a privacy agreement."