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, published research about 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.
In short, every key stroke you enter while on Facebook is tracked and can be analyzed. The research of self-censorship is interesting but clearly raises some concerns about privacy.
The research paper can be found here, I found it to be a very insightful read.