I always kept my LinkedIn profile strictly professional. It has always been reserved only for people I either had worked with in the past, or people I crossed paths in the Industry I belong.
I really liked the feature that you have to select the position and/or the company from the drop-down box, or have to actually add the job to your "LinkedIn Resume", and it's like a whole new story. I have a little over 100 contacts and never raced to become 500+ superstar. The 500 past and current colleagues? Gee, you're either a hell of a job hopper or had worked for huge corporations and added everyone from the executive team to the tech support people.
Some people even post their email address to the profile, since an email address is required to be added to your network. Also I often come across the message on personal blogs "I believe in networking power, connect with me on LinkedIn, my email address is firstname.lastname@example.org!"
Interesting enough, I came accross the List of Social Computing Strategists at Enterprise Corporations.
As you can see the majority of these Industry leaders have less than 300 LinkedIn contacts in their networks. And I am sure they receive dozens of connection requests daily and they know WAY many more people in their respective industries, and they do have MANY friends, but they chose to be selective and only add those they had a professional ties with.
To me LinkedIn profile were more like an addition to your resume and employment verification at the same time. I still get amused when HR department requires people fill out these obsolete employment applications with a last 3 places you worked and 3 references. Just look at my LinkedIn profile and please spare me from all these useless paperwork!
To my surprise LinkedIn claimed 47% of the UK's web users are mixing their social and professional lives by accepting networking invitations from "frolleagues" - basically from your friends or from the people you really had zero professional interaction, just know them socially outside the work.
It was nice to find out that the majority of LinkedIn users (73%) feel the same way and they wanted to keep them separate. However, 36% of workers feel an obligation to accept friend request from colleagues, even though they don't really have any professional relationship with.
"It's becoming increasingly important that we keep our professional and social lives separate and manage our online reputation as effectively as possible," said a LinkedIn spokeswoman.
So I would only add people I worked with, for everything else is a Facebook.