I used LinkedIn for a while, and it was cool at first. It's meant to help you find people with specific interests or skills within your broad business/social circle, so you can look for an "information architect" or "marketing expert", and find a resource that's also within a degree or two of someone you know well.
Pretty quickly, though, I ran into what's got to become a systemic problem on these things--someone I know got joined to some kind of mad link whore, who had like 1,000 1st-degree links.
Now all of a sudden, this guy's just two degrees away, and my "3rd-degree" network jumps to 1,200 people. Since half the people he links to are link whores as well, my fourth-degree network is like 10,000 people, and the whole thing is totally useless. Every search gets 500+ hits, and they're all through some guy I definitely don't want to know.
You know what I call the remaining list of people who are all within one or two degrees of connection to me? My friggin' address book.