I have been a linkedin user for the longest time. In fact, I was a member of linkedin even before I became a member of twitter or other social networking sites. No - I am not on Facebook. It's a pretty darn good website for professionals and peers to connect, discuss and share knowledge. It's also a good website for headhunters to find candidates for jobs and likewise for the professionals It's a good platform to create professional networking which no other social networking sites provides.
One thing I have noticed with LinkedIn - Users on that site have no qualms about connecting with strangers and accepting them on their face value, which in linkedin's case is their profile and experience. There is no noise about what is private and what is public. There are no privacy concerns which is generally associated with other social networking sites. Each user is free to connect with other users who chooses to make their profile open, so that anyone can connect with them. There are groups one can join. Most moderators can be pretty lax about someone joining a group. It really amazes me, that how people conceives one social networking site to another. At one they are perfectly open to connect with anyone on LinkedIn, but on the other hand, they have reservations and carefully choose who can be in their other social networking sites like Facebook, Twitter or Google+. I understand, they are two different networking sites catered to two different segments, but our behavior and our thinking is so different when it comes to what is personal and what is professional.