"Previously, for most people, it was set to Public"
So when I joined the default was I believe "Friends of Friends"... Then Facebook decided unilaterally to change loads of things to being Public (Oi! No!!)... Think this was around the time there were discussions about partnering with search engines...
Soon after that the new (current) privacy model came in "because the options were confusing" IIRC along with easily changing visibility on a post by post basis... Not sure if that was even possible before...
Of course I thought the original privacy options actually were clear enough, though every so often you felt you needed to check every couple of weeks in case FB changed your settings for you...
Ho hum...