Set up a separate email account that doesn’t use your real name and use that to register and receive mail from the site. That way if you want to stop using a site, you can simply drop that mail account.

Using a fraudulent identity to prevent identity fraud? Hmm. Interesting.

There are other alternatives, such as not using social networking sites at all (which may have the added benefit of increasing productivity). Moreover, no matter how careful one is, social networking sites themselves are still vulnerable to security breaches, e.g. SQL injections.