When I was a teenager, my sister had a party while the rest of the family was away and she was supposed to be staying at a friend's house. When I got home, I couldn't find my diary. It wasn't missing for long and I found it under my mattress (with a very unpleasant message scrawled across one of the pages), but while it was lost I was a nervous wreck, wondering who might be reading it. If, back then, someone had taken my diary the worst they could probably have done was read it out at school or, at a push, photocopy the odd bit. But now with all the social networking? Potential for huge embarrassment.