My life is relatively open, as can be seen from my many posts in Facebook and the internet. When one tries to lead an honest life, there's really nothing much to hide, anyway. Having said that, I don't believe anyone reveals everything. There are certain things that we don't say, simply because it involves other people, or even if it does not, it is not something that others want to hear.
Yet, I've met people who are very secretive about their life in Facebook. (They want to know about you, but they don't respond likewise.) Sometimes, they are just shy but at other times, they are so insecure that they believe that by being open, they may unwittingly exposed themselves to people who may take advantage or try to cheat them. Yet, there are also others who are secretive because they've ulterior motives and just want to take advantage of you. I've have met this type too.
(P/S: Would anyone go round telling people how much you've in their bank account? Many companies don't allow you to reveal your salary, either.)