It's good to be skeptical or even paranoid on the net. I say that as a former professional paranoid.
Linked in has a reasonable default set of privacy which you can raise or lower. Since it's about resumes and professional networking, the default is a little more open than sensible folks would use one rather more personal FaceBook profiles, and that is ok.
Linked In also has a more transparent business model on how they are monetizing your free service. They are on the give 99% free to build a community for which 1% will pay for advanced access. Anyone can buy access to fuller search results and to ability to send job inquiries - to those who indicate interest in such - to folks without being introduced. You don't have to build a better game to spam peoples friends, just buy recruiters level access. That's transparent, and in my interest - If I need a job later, I will want active hiring managers not in my network yet to be able to find me.
I wouldn't use LinkedIn for purely social networking, but I don't think I want to give professional references on dating-and-music sites like FaceBook or MySpace either.
I'm a Lisp variable -- bind me!