Isn't this pretty much what YCombinator startup Overhear.us is doing?
I think the alternatives are the intact networks that people use to find new jobs (alumni, friends, professionals, head hunters). How will it compete with Facebook?
Right now on Facebook, I can search by employer of interest (e.g. Google, McKinsey, etc) and it will pop up the closest contacts by some sort of FoaF distance algorithm. I can friend them and message them to open up a dialogue.