A researcher from a Dutch university is warning that Facebook's 'Like This' button is watching your every move.
Arnold Roosendaal, who is a doctoral candidate at the Tilburg University for Law, Technology and Society, warns that Facebook is tracking and tracing everyone, whether they use the social networking site or not.
Roosendaal says that Facebook's tentacles reach way beyond the confines of its own web sites and subscriber base because more and more third party sites are using the 'Like This' button and Facebook Connect.
But data about the user is sent to Facebook regardless of whether the Like button is actually activated.
Which is all quite scary - but not too surprising, given Facebook's reputation for snooping on its registered users.
What becomes really scary is realising how Facebook can track your movements even if you haven't signed up to its fake-friend collection service for lonely teens and sad divorcees.
Even if you don't have a Facebook account, you are far from immune from prying eyes, as Roosendaal explains:
"When a user does not have a Facebook account, there is no cookie and no user ID available. In this case, an HTTP GET request for the 'Like' button doesn't issue a cookie.
"However, when a site is visited which includes Facebook Connect, this application issues a cookie. From that moment on, visits to other websites which display the 'Like' button result in a request for the Like button from the Facebook server including the cookie."
Which means Facebook has swiped another batch of valuable data without asking for permission.
When you consider that 40 million unique visitors ended up on a site using Facebook Connect in a single month in March 2009, and that these particular cookies have a two-year expiry date, that ads up to a lot of user data flying around looking for a home.
"Based on the cookie, the entire web behaviour of an individual user can be followed," says Roosendaal. "Every site that includes some kind of Facebook content will initiate an interaction with the Facebook servers, disclosing information about the visited web site together with the cookie."
So you find yourself dragging all of this invisible data round with you like a piece of toilet paper stuck to your shoe, even though you have never even been to Facebook, let alone signed up.