Written for executives, this piece on networking has some good reasons why you should work on your network. I don't think it requires 100% commitment from the non-executive crowd, but it does require work on your part. As mentioned, be active, follow up with people, and don't force things.
Listen, and engage people in conversation. Get to know them, interact with their ideas, and present your own.
You can do this in person at any gathering, but also online. You can interact with people on Twitter, LinkedIn, etc.
Networking isn't hard, but it does require an effort.