It seems like there are quite a few people I know who use LinkedIn to communicate across their network, since I still get requests relatively regularly, where most of the other social networking tools have died out. They seem to still be adding capabilities to their product, so they must be doing OK.
I mentioned in an entry the other day that social analysis will be one of the IT approaches to addressing the knowledge transfer issues of the retiring workforce and succession planning ... and more active tools like LinkedIn or social extensions to semantic Web activities would be a good set of tools. Who enters and who consumes content are a very useful part of the context of the information. Some of the early tools that I have experimented with in the past have all gone belly up as well.