LinkedIn is fantastic if you want to look for a job or just be connected to various groups of people. I have been using LinkedIn since it's begininning; seems to be as relevant now as ever.
Whether you are a passive candidate, active candidate or general networker, LinkedIn is worth your time to fill out your profile and start getting connected.
Twitter is another story; little snack bite bits of information rushing across your desk on a minute by minute basis from the people you follow and of course from the information you share for the people who follow you. I have learned quite a bit from following some of my favorite recruiting folks on Twitter. It's important for me to add to my tool belt and the people I follow (my daughter included) are quite witty and brilliant and willing to share information.
The Ning system of Social Networks is fantastic; create your own Social Network or join others that suit your fancy or interest. I am partial to the MedxCentral Ning for all things medical related and all people in the medical field.