Facebook knows what you write on your status updates, comments or biographical info whether you publish it or not. Facebook has said that it is within its terms of service to know the status or comment even if the user has deleted that, i.e. “self-censorship”.
"We receive data about you whenever you use or are running Facebook, such as when you look at another person's timeline, send or receive a message, search for a friend or a Page, click on, view or otherwise interact with things .... ," the Data Use Policy says in the section titled "Information we receive and how it is used."
This report came from the study that checked the habits of about four million people in the U.S. and U.K. over 17 days in the summer of 2012. Study shows that nearly 71% of Facebook users cut their own status updates with 51% deleting at least one post and choosing to leave 4.52 unpublished on average over the period. Another 44% censored comments, with an average of 3.2 going unpublished.
Study shows that males remove their comments or posts more often than women and older users remove their contents more often than younger users.
Facebook can see what users type even if status is not posted - Los Angeles Times (http://goo.gl/fvNHkg)