The NSA is a topic of discussion on social media tonight because of 60 Minutes, but here’s something I bet you didn’t know about the incredibly intrusive techniques Facebook uses to monitor everything you do on their site (and beyond): Facebook Self-Censorship: What Happens to the Posts You Don’t Publish?

We spend a lot of time thinking about what to post on Facebook. Should you argue that political point your high school friend made? Do your friends really want to see yet another photo of your cat (or baby)? Most of us have, at one time or another, started writing something and then, probably wisely, changed our minds.

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.

Yes, Facebook is actually keeping track of the things you don’t post. The stuff you delete because you thought better of it. The stuff you thought was gone forever, bits lost in the ether. The stuff you didn’t want anyone to see.

Facebook sees it, and records it, and analyzes it.