The word "networking" often has a negative connotation to many people. Many feel that network is about smoozing and impersonal communication. Since coming to MIT Sloan, I have recognized the value of networking as a powerful career and professional development tool.
I am currently using LinkedIn to build a network of professional contacts to help each other with introductions, job search, and business opportunities. So what is LinkedIn? It is basically a social networking site like Friendster where you network with other people through a common contact. 
While you put social interests and favorite movies on Friendster, LinkedIn is a site geared to professionals. You see resumes, business interests, and organization affiliations on LinkedIn. If you are job seeker you can either scan for job openings or find out who's hiring from your network. You can also search for business opportunities from your network. Say for instance you want to set an import/export business in Germany but don't know anyone there; you can look up on your network for someone who can help. In fact, one of my classmates got 2 job offers by the 3rd week after joining LinkedIn while a friend of a friend was contacted by someone from his LinkedIn network with business opportunity in his home country.
The membership is free and so far I have not received any spam of any sort from the network. Unfortunately, for some of my friends, I have become somewhat of a spammer. I have been a little insistent on asking my friends to join LinkedIn. So for those of you would have been getting emails from me asking you to join LinkedIn, I apologize for the spam. Nonetheless, I highly recommend LinkedIn and it is a great tool to have if you want to network professionally.