Even as a non-Facebook user, if you were to only post on HackerNews all of the time, a lot could be inferred about you. Apparently, the value of sharing exceeds the cost in privacy. There are many types of thoughts of that people would share on the internet, some more revealing than others (e.g. "Today is Friday" vs. "I am 190lbs"). Some people must be very skilled at posting inert content which does not in any way tie to their personal lives, others may not be -- or simply don't care, or believe the sharing value to be very high, etc.

In terms of psychology, UI/UX design and engineered content could try to maximize the likelihood of getting a user to post emotionally-charged or revealing content.