People are used to dividing the world into broadcast media (television, newspapers) and point-to-point communication (the telephone, face-to-face communication). Because the Web has many aspects of broadcast media, people often talk about the information we put on social media sites as "public," as though posting on Facebook is like appearing on national television. In reality, most of what we do online falls in the second category.
We employ an wide variety of techniques and social conventions to control who we communicate with in the offline world. We might share details about our love life with friends at a bar that we wouldn't share over Thanksgiving dinner. Conversely, we might tell our families about medical or financial decisions we wouldn't discuss at a bar. And we lower our voices when we want to make sure the people at the next table don't overhear us.
The early Internet was very different. Users faced a stark choice between posting information on a public Web site or sending it in a private email, with little in between. The new generation of social media tools is helping to bridge the gap. Twitter lets me make my tweets public or limit access to people I've specifically approved. Facebook allows me to decide whether my profile will be visible to others with a princeton.edu email address, whether friends-of-friends will be able to see my photos, and even whether my profile will show up at all when someone searches for my name.
Of course, there's still a lot of room for improvement. Many users find these tools inconvenient or hard to use, and some are careless about posting information that could become embarrassing in the future. But we shouldn't be too impatient; the offline world has a centuries-long head start in developing privacy-preserving tools and social conventions.