Adam Kramer, a data scientist at Facebook, and Sauvik Das, a summer Facebook intern, tracked two things for the study: the HTML form element where users enter original status updates or upload content and the comment box that allows them to add to the discussion of things other people have posted. A “self-censored” update counted as an entry into either of those boxes of more than five characters that was typed out but not submitted for at least the next 10 minutes.
Interesting and clever approach, and completely understandable from Facebook's perspective. When and why people aren't posting is an important point for them to understand, just as a business might want to understand their conversion rate through various marketing efforts or their funnel abandonment.