When I was in school, I believed that I would get jobs through the simple purity of my ideas. The word "networking" was on a list in my mind with other words like "selling" and "schmoozing"--words that I'd heard were part of our business but that I had no interest in. It was all about the ideas.
Well, here's a secret. I have never sat in an interview and shown anyone my portfolio. Not once. I got an internship through my school, I got a job through my internship, and I got a second job through connections I made at my first job. I hope you think no less of me.
The math is simple (actually, it's probably complex, but the idea of it is simple). In advertising, the usual stint at an agency is probably 2-3 years. So take however many people you know from school, or the agency you interned at, and imagine them all bouncing from agency to agency every couple years, and all the people they come in contact with at those agencies. That's a lot of people. That's a lot of contacts.
I'm not that old, but when I left school there was no Facebook or LinkedIn or Twitter. We had email address books and phone numbers. It's easy to keep in touch with people these days, or at least keep track of where they are and what they're up to. If you're not using at least one of these sites, you're missing an opportunity that you may regret if you find yourself in the market for a new job. Because when we need to hire someone at my agency, the first thing we do isn't look through the stack of portfolios in the corner. It's not call a headhunter. It's ask everyone in the creative department: "Hey, you know any talented people looking for a job?"
Here's a good post on how to use social networks to find a job.
I got this link from a very talented writer I know, Tony. How do I know Tony? He is a former student of mine. We've stayed in touch via Facebook and Twitter.