Social networking websites should have nothing to do with your professional career, right? But it's not that simple. Anything you put out into the public domain could potentially affect you professionally (especially if you make a routine habit of adding co-workers to your Friends list on Facebook). Most firms, at the very least, will perform a routine Google search on your full name and click on a few of the links. It's easy and free due diligence for a firm wanting to make careful hiring decisions.
I recently wrote this article about ways to ensure that your online information doesn't come back to bite you during the interview process.