I’ve had a couple of unsolicited job offers from Linkedin, which I declined cos I’m happy where I am. I don’t use it that much but have built up a fair network of people mainly who I’ve worked with who are now spread across all different industires.
It’s a good way to keep in touch with people on a professional sense. I don’t have anyone I work with on Facebook, but I do on Linkedin.
Linkedin is work, Facebook is personal and the two don’t mix.