In response to the recent announcement by Facebook that it is entering the enterprise world, Joseph O’Connell has a  good post arguing that  corporate  social networking has in fact been around for a while, enabled by solution providers like SelectMinds.

The challenge to this strategy (if, indeed, it is Facebook’s strategy) is that many corporate social networks already exist.
SelectMinds has been developing online communities and the software
that powers them for over five years to help leading companies in the
consulting, accounting, investment banking, and legal industries where
the value of networking amongst knowledge workers is vital. While we
have several great features that help people connect, including "friend
of a friend" technology made famous by Friendster et. al., SelectMinds
celebrates the value in fostering relationship-building in person as
well as online.