Try this little thought experiment, sign up a new facebook account with a fake name and use a throw way gmail account, spend about 10 mins looking at the content, dont join anything. Never access it again.
Affter a few days you will start getting mails on that account suggesting friends that you probaly actualy know. This is because they have linked your temp account and your normal account via tracking of browser cookies, even if you use a completly private login to your system so that none of the browser cookies etc work, they track your ip address and the browser fingerprint.
Its all a bit creepy, and totaly without consent.