You read a lot these days about how young people are using social media recklessly and revealing their entire private lives to the world. I'm not sure and although I can't link to any conclusive research in this area I suspect that this problem is as relevant for the over 30s, if not more so. In fact I would claim that teenagers actually prefer relatively "closed" social networks to wide open ones and that their parents are the ones who share with everyone. Texting, Skype and MSN are the dominant means of communication and there you only communicate with friends. Many are wary of open apps like Facebook and Twitter since you don't know who will be reading (often parents and teachers!).
Contrary to popular belief, social networking appeals to all age groups. 64% of all Twitter users and 61% of Facebook users are 35 or older (Study: ages of social network users). It's a similar story in many other apps normally associated with the so-called net-generation. Most users are no doubt relatively unaware of their own security settings and that in many cases the default setting is that everything is public. You have to take the initiative if you want to restrict access. There's been a lot of public debate about how much information Facebook in particular owns about each one of us and what that information may be used for in the future. Read, for example, a recent blogpost Top ten reasons you should quit Facebook.
An article in the New York Times, The tell-all generation learns to keep things offline takes up the growing privacy concerns of many young people and confirms my suspicion that they are more concerned than their parents' generation:
"In the Pew study, to be released shortly, researchers interviewed 2,253 adults late last summer and found that people ages 18 to 29 were more apt to monitor privacy settings than older adults are, and they more often delete comments or remove their names from photos so they cannot be identified."
Basically you have to assume that whatever you publish will be seen by more people than you might wish and therefore be careful what you publish unless you're sure of the security settings. Maybe it's time to move some activities away from Facebook and suchlike and on to more secure or more relevant networks. Many people mix work and play on, say, Facebook, resulting in unfortunate consequences. Decide what you want to do on each network and who you want to communicate with and be consistent. Use different identities or even different networks (eg Facebook for private and LinkedIn for work) to avoid work contacts learning all about your family life and vice versa. This is nothing new really, it's just that we have new and exciting settings for interaction today and we have to relearn a few common sense principles.