You know those games on Facebook where people tag you and you have to answer a set of questions? It’s a great way for me to profile you a lot more. Combine that with other sources such as your interactions with other people within your social graph, some details I know about you, other social network profiles, I could profile you and your interests to a good degree.
Bonus points because you’re voluntarily giving that away.
It’s nothing new. Facebook, Google, and Twitter (but not limited to) does that in their analytics anyway, only with richer sources of information. That includes things that you click, and items being generally interacted with.