With so many people joining social networks like Facebook, MySpace, and LinkedIn, it's becoming harder and harder to protect your personal information. If one of your friends happens to add a host of different facebook apps, those apps will have access to his friends' (your) information. There is nothing you can do to stop this unless you either remove all your friends or create very limited profiles.
In addition, people have come to expect to be able to add you as a friend after they've met you and rejecting them may be construed as anti-social. Imagine a recruiter not being able to look at your information on LinkedIn or a potential date not being able to look at your interests or photos on facebook - you will be missing out on opportunities.
How is one supposed to play this game where you want your information both hidden and shared? My solution is to embrace this lack of privacy: integrate yourself into as many social networks as you can, start a blog, post on various forums, publish your photos on Flickr, and so forth. By being famous (if only on the internet) you will eliminate a lot of the adverse effects of having your information public. You will have enough of a community to support you in case anything goes wrong and you can stop worrying about your information being shared.
How often does Bill Gates worry about his identity being stolen?