Typically, socio-economic groups have tended to keep their business and social lives separated. You can no longer do this thanks to Facebook and it's network of members who decided for everyone that your social and business life are one and the same.
I certainly agree that Facebook capitalized at the right time and produced a modern communication medium that caters to the social existence of people.
But I strongly believe that business and pleasure activities need to be separated otherwise emotions can invade smart professional business decisions that should be decided logicallly instead of emotionally.
Linked In, on the other hand fills the void that would be left by Facebook if they chose to just support the social aspect of peoples lives.
Linked in provides opportunities for like minded professionals and people of similar ilks to interact professionally in an environment centered around business and employment.
I have been a member of LinKed In since it was founded. It is the original business networking site, in my opinion.
It would be fun to hear people opinions on this.