From the post: A few years ago, before the CEO cared a whit about social networking or identity, a Google User Experience researcher named Paul Adams created a slide deck called the Real Life Social Network. In a very long and well-illustrated talk, he makes the point that there is an impedence mismatch between what you share on facebook and your interactions in real life. So when you share a photo of yourself doing something crazy at a party, you don't intend for your aunt and uncle, workmates or casual acquaintances to see it.

In real life, the crazy party photo you would be showing your friend would be on your phone's screen or (in olden days) it would be a hardcopy photograph. Putting your phone or the hardcopy photo back into your pocket would be the end. The potential for blowback would be limited to whatever could be whipped up by your friend's verbal description of what he'd seen.

Whenever you put anything on the Internet, though, there's not really any way to control its spread. Whatever privacy mechanisms a sharing/social site puts in place will be broken by crackers at some point or part of the system will break on its own and leak information. Long before that, though, one of the people you permitted to access the controlled content will simply copy it and share it with other people.

Don't put anything online that you wouldn't be comfortable with your grandma or work colleagues/boss reading.