I've been a LinkedIn user for a while now, and I like to look around and see who people know, but it never had a big payoff. However, that all changed recently.
I was going through my address book and checking to see if I had linked to all my known contacts. I came across Aaron, who I worked with several years ago but no longer had a valid email address. I did a quick search and found out that he was in my network. I had my contact 'forward' the request on to him, and we got back in touch.
It is easy to lose track of people, and often hard to get back on touch.
One interesting side node to this. One of the bigger uses for LinkedIn is for recruiters to create huge networks and use it to identify resources. This is useful, but some people may feel that it is an abuse of the system. They don't REALLY know all their contacts well, they are just people they talked to at some point. However, people like this are useful to the system. When I reached out to Aaron, one of the links between us was a recruiter. People like this help 'grease the skids'.
Anyway, if nothing else the experience justified my further upkeep of my network.