The distinction between "public" and "private" is gray and messy when it comes to corporate social media platforms. Is anything really private? How do you send a message to someone in Messenger? You're not sending the HTTP request to person X. You're sending it to Facebook with metadata that says "please make this visible to person X and consider it private". Then Facebook keeps the message and decides whether or not to publish it onward to person X, and whether or not to display it to anyone else (internally, externally, in logs, etc.). It's not like a package that gets sent over to person X's house. It stays on Facebook's property at all times.

When you post something to "your" feed, you're sending it to Facebook with metadata that says "please post this on my wall" or whatever.

To strain an analog analogy, this is not like the telephone or even the post office, where you hand them something or send out voice packets and they just look at the recipient and forward it on to the actual person. Everything you send is sent to Facebook and kept at Facebook.

Replace "Facebook" above with any social media platform or cloud service. They're fundamentally the same. I don't consider iCloud photos private. You're sending your content to Apple, not putting it in some safe that you alone control.