May be you know or may be not, but your friends on Facebook could have authorized one or more apps to read part of your profile. Typically the list of your liked pages.
Actually, what was happening? While signing in a Facebook app, a user was able to authorize that app (is the app asked for) to read your liked pages or your wall. That because the user was saying to the app it can emulate him.
It can be reasonable until the app just diggs the friends information to show them to the user who authorized it, but apps can send such data to other servers. In this way a piece of a friends profile could be stored and used to classify that friends based on his preferences.
I know that because I worked on an app doing that. Since April 30, 2014, with the introduction of the Facebook Graph API 2.0 this is no more possible. Or, better, the old behavior will available untile April 30, 2015, but only for apps created before April 30, 2014.
With Facebook API 2.0 is no more possible for an app to access information of you friends on Facebook. This is an important change… even if when you open the wall of a user with your browser you can still see his liked pages. I wonder if that behavior will be rectified as well!