Ask HN: Pictures on fb and Google+ public by default

Below every album on fb I'm shown a public link to share the album so for the (unlikely) case that someone "guesses" this url he can see all my albums if I have shared them before or not. Is there any way to make sure that there is no public url?

If I share a picture or album on Google+ with someone who isn't yet on Google+ (sharing via email) this person will be provided with a public link as well. Does anyone know if this link will be generated only when I share something as described above or are there public links to any album/picture by default anyway?