Is LinkedIn different than Facebook?

I recently was invited to join LinkedIn by a freelance writer who I respect. I think I might move forward and sign up, but I hesitate.

I wonder:

1. Is LinkedIn just another social networking site?
2. What makes LinkedIn different from Facebook and MySpace?
3. What benefits does LinkedIn offer?
   

Most people know that I tried Facebook and then deactivated my account because I couldn't find much value in it. Will joining LinkedIn be a deja vu experience?