HN Commenters: what do you feel is the moral responsibility of a social site in an instance where someone has the ability to post something at large vs a group

HN Commenters: what do you feel is the moral responsibility of a social site in an instance where someone has the ability to post something at large vs a group of 'friends'? Do you think the site should notify a person at a per post level/status entry (some type of warning... 'this will be posted to ALL site X users, etc') or just a one time warning at the personal configuration level?