Some recruiters and hiring managers just can't get enough information about a job candidate. And the restrictions on what you can - and cannot - ask in the interviewing process are restrictive.
So therefore, there is no reason for any employer to search Facebook, MySpace, etc. for further information on a candidate. Sure, you might find more information - but it may or may not be true. And if you find information that you're not supposed to know, then you could be liable for a wrongful employment decision - whether you used that information or not.
Just say no to Facebook when hiring an employee.
Jackie Ford at Marketwatch has a good article which articulates specific, additional reasons why employers can be liable.