Ok, this one is really getting scary... and a further reminder to minimise your (known) online presence as much as you possibly can:
Schneier on Security: Monitoring Employees' Online Behavior
The article is about a company that provides software/services to swarm all over a persons online presence on social networking sites (Facebook, Twitter, LinkedIn, even Flickr!) to try and summarise your personality and life! They sell this service to potential employers to use on job candidates or existing employees, with this sort of scare story:
Now the other option to trying to reduce your online footprint is to go for the overload approach: Create a hundred or so Facebook, Twitter, MySpace, etc accounts, put absolute crap in them, fill them full of all the weirdest stuff you can think of (along with some apparent completely normal and boring ones). Make sure they all are close enough in name/age/location to yourself, and see what happens