Another point: Facebook is undergoing a big resurgence in the 30+ year crowd. My wife is using it to reconnect to HS friends, I'm finding Fraternity Brothers there. So it is definitely more social than LinkedIn. In LinkedIn, your professional profile is open to anyone to see (here's mine for example:
www.linkedin.com/in/dplumley). So make it full of information. Facebook can be (and should be) limited to friends only. But it is still a form of networking. It just needs to be more subtle.
Don't forget, there is a PPOT group on LinkedIn.