That's not actually correct history. Facebook started with most data visible only to your friends, and gradually increased the visibility (given default settings) over time. You can track the changes here:
http://mattmckeon.com/facebook-privacy/
Also, when Facebook launched it was exclusive to Harvard, and then to Ivy League colleges, and then to all colleges, and even for a while after they opened it up it was still mostly young people. Your friends list was a closer match to your peer group. So it's possible that the audience people used to get with a friends-only post a few years ago isn't all that different from the audience you get with a secret group today. I remember when I was first finding out about Facebook around early 2009, it had the distinct feel of a private social club.
The only difference was that G+ started with private circles, and later added circles which could be shared and public communities, whereas Facebook started with the "let it all hang out" model and then grafted privacy on later. The problem with this, though, as Randi Zuckerberg discovered to her discomfort, is that like security, grafting privacy onto a product design afterwards is fraught with peril and often doesn't work well (if at all).