Quote From mlinsell:
I would say Facebook is almost essential now for networking with over half the UK's Internet users accessing it weekly according to Mintel.
Bit of a leap of logic there - just because half of UK internet users access it weekly doesn't mean it has any value to someone in a professional context at all.
If you're concerned with online netowrking I'd stick with the specialist sites other people mention. I don't use them but I'd consider them a more appropriate tool than Facebook.