This has become the crux of my objection to social networks: they exist to gather data on you. Whatever benefit you get from it, their purpose of existence is data mining.
As the Pinboard Blog puts it, that's ANTI-social.
"We're used to talking about how disturbing this in the context of privacy, but it's worth pointing out how weirdly unsocial it is, too. How are you supposed to feel at home when you know a place is full of one-way mirrors?"
http://blog.pinboard.in/2011/11/the_social_graph_is_neither/