facebook privacy

I was surprised how the privacy setting on facebook will change without you changing it.  I do like the feature where I can post a picture that will only be seen by myself and noone else.  I wonder if someone really smart can see that picture though?  I am working on updating my privacy setting for facebook and trying to set up a twitter account.  My daughter set me up with snapchat, but I don't know how to use it besides sending pictures and stuff back to her.  I was not aware that it needed a privacy setting in order to not share my phone number.