I think at an early point, about when Facebook was in the "open to all university students" phase, there wasn't even a private option.
I have a very different memory of my time on Facebook 10 or so years ago... It felt like every two weeks some update would change my settings to "public" in some way.
On Facebook, public isn't the default, but on Instagram it is. All those billions of photos, including all those famous people: evidently fair game.
I don't think you can make a blanket statement like this - the defaults and privacy policy for FB have changed a lot in 16 years, multiple times. It might be the case today, but this is 16 years of data.