[R]atio of postings per active public group to postings per active private group [is 2:9] . To illustrate that defaults matter, in our original design where users had to explicitly make photos public (and couldn’t switch them back), the percentage of public photos in the system hovered around 45%. In the new system where you can set a default for photo privacy settings and the default ‘default’ is public (and you can change the setting at any time) just over 85% of the photos are public.
The bottom line here is that posting public photos are opt-out. While Facebook has had some harsh reactions towards most of their new social features being opt-out, I don't feel the same concerns with Flickr. I guess this is what the Snapr culture [PDF] is all about. Either that or Flickr is doing a much better job at making these options and settings transparent and making such transparencies obvious to the user at the right times (in-task?).