Steemit. As a public social platform. For now I doubt there is any social platform that is well developed enough to be safe for creating a private space to share more intimate and private details about your life with those people that you allow as friends as in FB to view your profile.
All of the platforms I know, are based on the principle that it is your very data that creates the funds to keep those platforms running and inherently that will never allow for true privacy and safety of your more personal and confidential information.
This is the number one reason why most of the free service providers, ask to have such unlimited access to your mobile devices, even to the point that phone microphones listen on our convos even when they shouldnt be recording and using that data for targeted ads.
All these "free" services do not cost cash, but information which they exchange for cash with governments and corporations.
If someone created a possibility of safe private spaces on steemit that somehow links accounts through a similar access key as one uses to login etc, I could imagine you could develop a secure platform for such uses like creating a social media site, which allows for more intimate exchanges with other close friends and people one knows or connects with.