The researchers now claim that Facebook tracks computers of users without their consent, whether they are logged in to Facebook or not, and even if they are not registered users of the site or explicitly opt out in Europe. Facebook tracks users in order to target advertising.

The issue revolves around Facebook’s use of its social plugins such as the “Like” button, which has been placed on more than 13m sites including health and government sites.

Facebook places tracking cookies on users’ computers if they visit any page on the facebook.com domain, including fan pages or other pages that do not require a Facebook account to visit.

When a user visits a third-party site that carries one of Facebook’s social plug-ins, it detects and sends the tracking cookies back to Facebook - even if the user does not interact with the Like button, Facebook Login or other extension of the social media site.

Opt-out mechanism actually enables tracking for the non-tracked

The researchers also analysed the opt-out mechanism used by Facebook and many other internet companies including Google and Microsoft.

Users wanting to opt out of behavioural tracking are directed to sites run by the Digital Advertising Alliance in the US, Digital Advertising Alliance of Canada in Canada or the European Digital Advertising Alliance in the EU, each of which allow bulk opting-out from 100 companies.

But the researchers discovered that far from opting out of tracking, Facebook places a new cookie on the computers of users who have not been tracked before.

“If people who are not being tracked by Facebook use the ‘opt out’ mechanism proposed for the EU, Facebook places a long-term, uniquely identifying cookie, which can be used to track them for the next two years,” explained Günes Acar from Cosic, who also co-wrote the report. “What’s more, we found that Facebook does not place any long-term identifying cookie on the opt-out sites suggested by Facebook for US and Canadian users.”