If you use Facebook or Google, you are essentially consenting to them knowing your likes, dislikes and even, sometimes, sexuality.
We've all been mildly creeped out when we get an advert on the side of our screen that seems to know a piece of private information about us.
Because the companies haven't always been that clear and transparent about exactly what they know about us and which parts of our data they're using for targeted advertising, people were surprised to see that they were "diamond sponsors" for an event on privacy hosted by the University of Amsterdam.
A better pic of the sponsors of the Amsterdam #privacyweek. This is so Orwellian I don't even. #APC2015 pic.twitter.com/bWfvq0oJ74
— Ancilla Tilia (@ncilla)
October 25, 2015
Some people have gone even further and said that the way in which the sites build up profiles of us using our information is a form of corporate surveillance.
Technology writer Sidney Vollmer said: "It is an oft cited example of the asymmetrical nature of the problem of civil rights in the digital age: Mark Zuckerberg asks us for as much data as he can, yet, for thirty million dollars, buys all the houses around his own in Palo Alto to get more privacy.
"By having Facebook as your diamond sponsor you offer Facebook a diamond chance at rehabilitation and respectability.
"It is either a sign of my lack of creativity or a sign of your organization’s faulty practice in this matter that I can not think of a worse sponsor for your much, much needed event on privacy."
Sorry, but this is a joke, right? Right!? #payforitownit. #APC2015 pic.twitter.com/ZBRU7NiUmB
— Sidney Vollmer (@SidneyVollmer)
October 24, 2015
Privacy advocate Aral Balkan told Motherboard: "We would not be having this discussion if Marlboro was sponsoring a conference on lung cancer,
"They just wouldn't be allowed to. Because it is clearly a ridiculous conflict of interest.
"The only reason we are even having this conversation is because we still don’t understand that Facebook and Google are to privacy what smoking is to lung cancer,"
"Corporations like Facebook and Google are in the business of people farming,
"The value they create is directly linked to the amount of information they have about you... So the one thing they cannot do is to compete on privacy. They can only compete on the illusion of privacy. And that’s the narrative that they are spending heavily to create."
Facebook came under fire earlier this year, when 25,000 people argued that the company illegally collected users' data.
The €500 (£354) being sought by each claimant means Facebook would be saddled with a €12.5m bill and a major reputational blow should the case, led by the Austrian law student Max Schrems, be successful.
Mr Schrems alleges that Facebook illegally tracked users’ browsing habits via software installed on other web pages, and participated in “Prism”, the American spy programme, among other violations.
The entire country of Belgium also tried to take Facebook to court for breaking EU rules on data collection and privacy.
