The social networking phenomenon, in the shape of sites such as Twitter and Facebook, has changed the way that the world communicates. You can argue, of course, that email already did this 20 years ago, but there's one (at least) fundamental difference between email and today's social networking. With social networking, everything you say is public, and stays around for a long time. Long enough for something you said a long time ago to come back and bite you.
Which is why, when you apply for a job with a company, there's a good chance that the first thing they'll do is to type your name into Google or Twitter or Facebook. It's a much better way to find out the truth about someone's personality, rather than reading their CV. Even if it is strictly against the employment legislation in most countries.
In some professions, what you say and do online in public is even more important than others. I was talking recently to a senior police officer, for example. He was pointing out to new recruits that their chances of getting into the "exciting" world of undercover work would be dashed forever if they had a Facebook profile that included their real name and photo. Even if that profile was created before they joined the police.
All of which brings us to a system called the Facebook Time Machine. It's a web site that you can access at http://fbtimemachine.appspot.com. It's free, though you'll need a FaceBook account of your own in order to log in.
The FB Time Machine is, simply, a tool for digging up other people's postings. Type in the name of one of your existing FaceBook friends, and it'll show you every message that they've ever posted on the system.
There's no doubt that such a tool is useful and fun. As to whether it makes the whole online world a better, safer place, I still haven't quite made up my mind.