I reckon people who say that Facebook is for friends and Linkedin is for business are missing out on the wider changes in attitude to work and business that the social web is enabling.
The web is all about people. It's about connections, relationships and energy. So is doing business.
Energy and connections are what help you do your work. Facebook, for all its faults, is good at increasing both. Linkedin in, in contrast, is full of people in suits being "professional".
The full impact of the social web will only be apparent when being "professional" - in the sense of restrained, impersonal, guarded and "businesslike" as distinct from being bloody good at your job - is finally seen as unproductive, dysfunctional, often bullying in intent, and a waste of time and energy.