Why? Because for most Facebook posts, the default privacy setting will be whatever it was for your last post—meaning that if you changed the “audience” for a recent batch of uploaded photos to “Public,” your next status update will default to “Public,” too.
But what about the “Control Your Default Privacy” on the Privacy Settings control panel? Well, that setting only applies to posts from Facebook apps that lack those little pull-down privacy menus (a.k.a., “audience selectors”) that you’ll find on the main Facebook site, or on the Facebook apps for iPhone and Android.
So, how can you permanently change the default privacy setting for all your future posts? Unfortunately, you can’t.
Instead, you should to double-check the privacy pull-down menu for each of your updates before you click the “Post” button (and yes, this includes the Facebook apps for iPhone, Android, and other smartphones)—annoying, I know, but better safe than sorry.
And if you’re worried that you may have posted a bunch of updates publicly when you didn’t really mean to, you can always go back and change the privacy settings for all your previous posts, en masse.