They conducted a series of tests including one where they did a Google search for the term facebook data policy. It led them to the Facebook data policy page which placed the datr cookie on their browser.
They then visited a Belgian website related to prostate cancer treatment which includes a Facebook like button and found that the datr cookie was sent to Facebook.
There was no formal notice regarding any cookie being stored.
Even without using cookies, a user's browsing habits can be tracked as web browsers can be identified by various ways to render each user's browser (and hence browsing trail) to be unique. To read more about this read Panopticlick.
To keep such sites from harnessing your data, here are some options:
- Use Tor Browser when possible
- Use Firefox with plugins*: Adblock Edge, Privacy Badger, HTTPS Everywhere, and NoScript.
- Use Firefox with above plugins in a Linux VM
* - the list of plugins is not at all exhaustive, you are advised to do your research and choose which ones are required for your case