I was glad to learn about LinkedIn, which seems like a very powerful networking tool. LinkedIn seems like a nice option for people who want to keep their personal and professional online lives separate. Personal contacts live on Facebook and professional contacts stay safely on LinkedIn.
I have 5 connections so far, though 2 of them said that they almost never look at the site! And this article, LinkedIn launches social news product mentions that a "substantial majority of members don't visit the site on a monthly basis." If few people are going to the site, however, maybe not that much networking really goes on there.