Okay, let's say you've joined LinkedIn. (You have--haven't you?)
You've set up a profile, polished your prose, and connected with a handful of friends (including me--right?)
So... how does this work? Do potential employers just stumble upon your profile and scream "Eureka!"?
Obviously not. You have to get out there (virtually) and get involved. Join some groups. Don't just join--participate. (Remember that part of your grade that was called "class participation"? This is what it was preparing you for.)
After you've joined groups and gotten a sense for the kinds of discussion that goes on there--jump in. Ask questions; offer information; find out more about the kind of industry you would like to work in. And yes, maybe you will meet some folks who will have something you want (a job)--and maybe not.
Above are three great links about how you can take your LinkedIn profile to the next level.