I had the same dilemma. People harp on networking, however, I would never refer someone I didn't know personally. I've found that *professional* social networking sites like LinkedIn (
www.linkedin.com) let you connect --and reconnect-- with friends and previous/current co-workers.
Since you would be a part of their network, you're able to tap into their network through recommendations and personal introductions. LinkedIn also lets you find former classmates, professors, etc., who can vouch for your work and your skills.
If you want to present yourself as professionally as possible, try avoiding social networking sites like MySpace, Facebook and Friendster. You can use those sites, but be EXTREMELY careful about what you post, and who you accept as a friend...employers frequently haunt those sites to weed out candidates. Unethical? Yes. True? Yes.