Everyone knows social networks exist to mine your data. What you post, who you post to, what you're talking about, and what you like and dislike are currency offered in exchange for the opportunity to interact with other people in real time on their servers.

But it doesn't stop there. Facebook wants to know what you don't post and why you choose not to.

Unfortunately, the code that powers Facebook still knows what you typed—even if you decide not to publish it. It turns out that the things you explicitly choose not to share aren't entirely private. Facebook calls these unposted thoughts "self-censorship," and insights into how it collects these nonposts can be found in a recent paper written by two Facebookers. Sauvik Das, a Ph.D. student at Carnegie Mellon and summer software engineer intern at Facebook, and Adam Kramer, a Facebook data scientist, have put online an article presenting their study of the self-censorship behavior collected from 5 million English-speaking Facebook users. It reveals a lot about how Facebook monitors our unshared thoughts and what it thinks about them. The study examined aborted status updates, posts on other people's timelines, and comments on others' posts. To collect the text you type, Facebook sends code to your browser. That code automatically analyzes what you type into any text box and reports metadata back to Facebook.

The study was pretty creepy to read, mostly because if one is self-censoring, there's an implicit assumption that what they just wrote isn't being read by anyone but themselves.

Farther into the study, the authors conclude that those most likely to self-censor are those who have friends crossing many different boundaries. Duh. My biggest complaint about Facebook is that it mashes up all of my social contacts from birth through death. My high school friends might not appreciate my politics, and my conservative family members certainly don't. So of course there's an element of self-censorship there, because alienating family members over politics isn't something I want to do face-to-face or online.

Just remember when you worry about the NSA watching you that it's not just the NSA. Big Facebook wants to know everything about you, not because they think you're a national security threat, but because you and your life are a marketing opportunity.