Good questions: check colleagues' info at start of new gigs, many people from my working past have contacted me through LinkedIn, and the email traffic on a few interest groups is low volume and of reasonably good value.

I have never used LinkedIn for its original function: finding a contact point in a company through a mutual connection. The reason for this is that it is so easy to simply email someone, introduce yourself, and try to say something interesting enough to them that they will spend a few minutes getting acquainted.