I've been using LinkedIn for over 2 years. Here are some other great features:

Import your address books - if an imported email shows as having a LinkedIn profile, send them an invite. If not, send them an introduction to LinkedIn.

The Q&A section is very valuable. I have posted a few questions and received some really helpful responses with links to more resources. Users are encouraged to answer questions by the practice of posters to give a Good or Best (Expert) answer accolade after their question closes. Questions are categorized and having "Expertise in" Graphic Design, Web Design, Education, etc... adds to the street cred of your profile – so I try and aswer any questions in which I have something to add. One can even suggest an Expert that may be able to answer the posed question. Another advantage to posting and answering questions is the ability to have "side" correspondence with a user you previously haven't known. This could pave the way to inviting each other to become a connection. The Q&A categories have RSS feeds. I watch them throughout the day and if I see a question I want to give input or that I want to learn the answer I go to that URL. It’s amazing how much I have learned through the Q&A feature.

Another great way to increase your number of qualified connections is to join one of the many Groups in LinkedIn. Usually you have to be accepted into the group by the Group's admin, but once in, you get a list of its members (which are of your same interest) and by initiating correspondence most likely a connection will ensue. And by being in Groups others are better equipped to find you.

Make it a habit of asking new colleagues if they are in LinkedIn.

Well, are you?

I've used the "find a colleague" tool that LinkedIn offers with limited success. It is helpful for getting a few connections and provides a place to start from.