LinkedIn going the platform route in response to Facebook?
I think they're going to have to change the perception of what LinkedIn is quite dramatically to get away with this. LinkedIn isn't an online place the way that Facebook or Tribe or MySpace are. It's organized around part of your life which is offline : namely your CV, your job, the company you work for and your (ex) colleagues. Its strength is letting you keep track of these things, while otherwise staying out of sight and out of mind.
Simply opening up the LinkedIn API isn't really going to do much unless someone can dream up some widgets that somehow engage this offline life.