LinkedIn is a professional network with tremendous power as a virtual rolodex of business cards for millions of people. LinkedIn helps people find jobs and jobs find people. Essentially not only is your business card on LinkedIn, but most of your resume as well.
Facebook is a true social platform. Facebook is designed to let people share and communicate and this they do at mass volumes each and every day! Facebook connections tend to be with people you know quite well and with whom you communicate on a more personal if not intimate level.
As you can see, these two popular social networking big wigs are quite different from one another and users can have the ability to pick either or both for their social networking.