I've been a user of LinkedIn for years. And, while I won't call it indispensable, I have found it quite useful.
But in the last two weeks, I've gotten more invitations to join others' LinkedIn networks than I got all of last year.
I believe this means something, but I'm not sure what. Most of the people who have pinged me are employed - so it isn't as simple as a declining economy driving a need to network. Several are European contacts - so perhaps LinkedIn is making more penetration there.
I don't know...