No, I don't think social networking is a "must do" scenario for nurses. I'm sure it can't hurt, but, there is nothing like the face-to-face aspect of networking (or voice-to-voice, if you're a phone person!). Besides, there are many cases where social networking has become a detriment to a person looking for a job, simply because it can be easy for a potential employer to form an opinion of you based on any public profiles you may have. All things in moderation, with common sense.
However, social networking isn't the worst idea, either. You can learn about new job opportunities, job fairs, and even engage others in conversations that could lead to jobs. Look at us here, for example. There are several people I've contacted with questions that have either helped me, or I've passed the advice to someone looking for a job in a particular specialty. It goes both ways