There were two diaries published in regards to whether Facebook is receiving your data before you hit "Post". I signed up for a DailyKOS account to write this diary to counter all the misinformation contained in this diary (Facebook tracks what I don't even publish!" No. No, it doesn't.) All I can say is that diarist got it 100% wrong and did not disprove the other diary as he claimed. I sent him a message first to try to get him to correct the misinformation, but he did not reply. FACEBOOK RECEIVES EVERYTHING YOU TYPE ALMOST AS FAST AS YOU TYPE IT, WHETHER YOU PUBLISH IT OR NOT. I have no idea if Facebook is storing the unpublished data, but they are receiving it almost as fast as you type it.
In fact, the easiest way to think of it for non-technical people is like the auto-complete feature in Google. As you type a search term into google, your browser sends that data to google for a list of possible matches and google returns them to you. When you type in the Facebook status update box, your browser sends that data to Facebook to look for a possible match against your list of friends names or pages you have liked. When it finds a match it will provide you a drop down box to select from so you can tag that person or page in your post.
Follow me to the extended section for the detailed proof.
Say I have friends named James & Jane and pages I liked for Janes Addiction and Jambase. Facebook will build an auto complete list with those 4 entries to choose from RIGHT AFTER I TYPE JA in my status update box. The list of 2 friends and 2 pages for me to choose from was built after my browser sent Ja to Facebook's servers.
If I then type m it limits my choices to James and Jambase. If instead I type in n it limits my choices to Jane and Janes Addiction. In the following example all I did was type the word New into my browser and Facebook sent my data off to the following Facebook TypeAhead/Search.php URL (https://www.facebook.com/....) with the ... representing the rest of the parameters that are not relevant to this discussion.
As soon as I hit the W Facebook provided me the option to select "The New England Patriots." That was the only match to the word "New" in my list of friends and pages I had already liked. Facebook did return some other pages that began with "New" in the results you don't actually see, but since I hadn't liked any of those pages previously they were not given to me as options to choose. I have seen on other occasions that Facebook will show me some possible matches to pages I have not liked, but I don't know how that algorithm works.
The following image and data was captured using the Firebug plug in for Firefox. You can clearly see the undeniable proof that my data was sent to Facebook without ever pressing post. The section with the 3 red squares are the pieces of data that was sent to Facebook and the section with the green square are the results that were returned from the Facebook Servers in the TypeAhead/Search.php Request
One of the parameters sent to Facebook does say topics_limited_autosuggest which I think means the results in this case are just there as suggestions for the browser to use to build an autocomplete list. It could be that page is also used to query other results, but I am not sure and don't need to look into it for this example.
Finally, it could be that Facebook permanently stores this unpublished data every time it is sent to the server. Or maybe they only save the data until you abandon the post or just until you log out of your Facebook session. I have no idea what Facebook does after it receives your unpublished data; I only know that Facebook is sending every word you type to their servers whether you publish it or not.