I was initially skeptical of networking sites, but I've become a big fan of LinkedIn.
This service allows you to find people through your existing network. For example, if you are looking for a marketing executive who works at Verizon, you need only tell that info to LinkedIn, and they will search your existing contacts and their contacts to find someone for you. E.g., you might not know that one of your colleagues is already friends with a marketing executive at Verizon.
This service has been very helpful to me. I've used it to find people to hire, as well as to find people to backchannel other people that I am considering hiring.
If you know me, and want to connect on LinkedIn, just send me an invite from your LinkedIn account. (If you don't know me, please don't send me an invite, as I don't like using any online network for "spam".)
While online networks have the potential for misuse, I've come to trust the folks at LinkedIn. They seem to "get it" with respect to their important responsibility for protecting member privacy.
Update 09may07: I am now going to maintain my LinkedIn feedback.