If you choose to tell me, even without really trying, I can know everything about you. If you choose to tell, we can all know what you had for dinner, what time you go to bed, what colour your hair was 3 years ago and the real reason you were late for work yesterday.
If you choose to put this information on Facebook, Twitter, your blog or out there some other way, a simple Google search will find it - or may not even be required, if we are connected in these spaces so that I get notified every time you post.
Yes, there are also ways this information is posted with less action on your part - newspaper articles, other people's blog posts - but for most of us, sharing about ourselves is a choice. The difficulty this leads to, as I see it, is not privacy necessarily (though this is important - guard your information and don't share more than you would want a stranger to know). Rather, one large challenge is to ensure that the person you portray yourself to be - kind, professional, jealous, hard-core partier, etc - must be congruent across each of these channels. And more than that, must be congruent with the person you are in real life.
Email your office to say you're sick and staying in bed today? Well then you'd not only better watch that you don't see any colleagues at the grocery store, but also ensure that you're not posting about a hard workout or great lunch with a friend on any social media.
Or, just had a major fight with a friend? Best not to air out your frustration in Facebook statuses and tweets... not only might that friend see your posts, but other friends will begin to wonder what you might post about them.
Social media need not be scary, or invasive. But just like any other communication, be careful with. One rule of thumb to consider: if you wouldn't say it to friends in a coffee shop loud enough for strangers to hear, then don't post it online, anywhere.