I find that a lot of people define 'using' LinkedIn as having a profile, but not a lot of connections. As a result, I think it's worth pointing out that the more you expand your network the more valuable the service will be for you.

Why you should expand your network on LinkedIn:

As you add more contacts:

1. You will have a larger database of people to search through when you are looking for a contact.  When you search for a person or a background you can only see contacts in your three degrees or contacts - more contacts means more people in your searchable pool.
2. More people will be able to find you when they search for someone with your background.