LinkedIn has done a good job of creating a social network for the professional aspect of your life. You can have co-workers who you enjoy working with, respect, like, but they still might not really be your friends.

But I think LinkedIn's success is partly because Facebook hasn't solved the social context issue. Work vs. friends is very important, but there are also differences in how we act with close friends vs acquaintances, friends vs. family, or even close friends that we met at different points in our lives. That would be a very difficult problem to solve cleanly, are there any social networks that are trying?