It is the nature of the web that everything can be accessed by anyone - assuming they have the right tools to do so. But let's just think about social media on a small scale to begin with. Just what you and I would use in day-to-day life. Many social networking tools allow you to fiddle with your privacy settings. Most of the time you can decide who sees what you post: you can have secret groups on Facebook and private boards on Pinterest.
What you can't control, however, is what the people you do choose to share with then do with your information.
As soon as you click that button you are releasing a piece of information about yourself to the world. Perhaps one person you do know shares it, then someone they know sees it and alters it a little, and then it gets posted to a different platform and altered again.
Don't get me wrong, web 2.0 and this process of sharing has led to all sorts of fantastic things, but generally speaking these are organised and intended to 'go viral'.
So I guess there are two ways to deal with this:
Only share your personal information with people you are 100% sure won't do anything with that information.
If you're going to share things with the whole world do a double check and see if there is anyone in your life you wouldn't like to see it. Your boss? Grandmother? An identity thief? If you can think of one then just don't share it!
This guide from PC world gives you a few helpful and practical tips for different social media platforms. I think I will be going to review all my privacy settings right now!