Let's use Facebook as an example. People use it to publish public event info, discuss work related issues and "like" whatever campaign they're into right now. But they ALSO send private messages and share private pictures in photo albums. Even _if_ Facebook was a perfect, non-crackable safe - that _behavioural_ effect merges the cautioness of public posts with the more open-minded private part. Causing all the never-ending comment-threads on junk that people continue flaming on with complete strangers much more often than had it been an explicitly public forum.
cc: @rozzin @bobjonkman