This has nothing to do with 'social media'.
This has nothing to do with 'social media'.
It may never reach other circles of your friends. In my own case, the circle of my FB friends that I know from technology do not overlap with the circle of friends that I know from sports very much. Something could easily go around one circle and not the other.
But FB merges them: Something on my wall is visible of all of my circles of friends. That's what I took from the article: Social Media kicks down the walls between circles of friends.
I still think this is way out of proportion. Let's use the example of your two circles. Assuming a few/some/most/all of your friends are in the same geographical area, how common is it to run into people from group A while out with group B (and thus open the possibility of group A people making comments you don't want group B to hear)? I know for me situations like this are very common. Just the other day I was leaving a restaurant with friends and ran into other completely unrelated friends who were sitting on the patio.
I don't think that just because Facebook is an easier way for your clueless/inconsiderate friends to let stuff slip is Facebook's problem, is all I'm saying. Facebook is just the highest profile, most widely used method of having a persona online. That comes with upsides and downsides -- if you aren't comfortable with the risks you can simply not partake.
One of the other important points to note is that people often have different circles of friends, who may or may not intersect, and even if you control what one group of friends knows, you can't control whether the other group of friends happens to let something slip. A situation that likely wouldn't arise IRL could happen due to the Kevin Bacon degrees of separation rule (sometimes a group of friends will intersect another group through the most bizarre circumstances).
edit: I took too long to type this out obviously. Ha.
Before social media, your siblings likely wouldn't hear a conversation between two of your friends when your name is mentioned.