So your friend invited you to join LinkedIn, and you signed up. Now what?
Answer: find out which of your friends and colleagues are already signed up. Here’s how to do it.
1. Export your contacts from your e-mail programs, into comma-delimited files. You can do this whether you use Outlook, Lotus Notes, Thunderbird, or any of the popular online e-mail services such as Yahoo, Hotmail, Gmail, and so on.
2. Import your contacts from these comma-delimited files into LinkedIn. To do this, log into LinkedIn and click My Contacts > Other Contacts > Add More Contacts > Other Address Book. That will put you on the Upload Your Contacts File page. Click Browse and select the files you created in step 1 (you’ll have to repeat this procedure for each file you created).
3. LinkedIn will then give you an opportunity to automatically send a LinkedIn invite to each of your contacts that are already LinkedIn members. Chances are, most of your colleagues who use LinkedIn already will readily accept your invitation.
This will give you a great jumpstart for adding people you are connected to via LinkedIn.
Peter H. Gregory 
Or not.
I actually pulled out of LinkedIn a couple of years ago. I felt my contact network is too valuable to share. No way I was going to enter ALL my best contacts into the system. No manage all the invites and endorsements and changes. I was deriving basically no value from LinkedIn, other than the ability to poke around and check into what stranger’s resumes and do some competitive research (or social engineering). Then I realized, they could be doing the same to me.
This guy has even more to say on it:
http://oracleappslab.com/2007/08/28/people-trust-and-content/
And this is even better:
http://www.oreillynet.com/onlamp/blog/2007/08/social_engineering_social_netw.html
Dunsany is a friend and colleague of mine, who’s gone underground, at least in terms of LinkedIn. Dunsany makes a good point. For this reason, I did recently remove several LinkedIn contacts from my list of contacts. I also changed my LinkedIn profile so that my list of contacts is no longer visible, even to my contacts. And I might take out some of the details about my background and activities.
I am still bullish on LinkedIn, but I definitely see abuses and will think twice about its value and potential risks.
Peter