What is Enterprise Social Networking?

Legal Definition
Enterprise social networking focuses on the use of online social networks or social relations among people who share business interests and/or activities. Enterprise social networking is often a facility of enterprise social software (regarded as a primary component of Enterprise 2.0), which is essentially social software used in "enterprise" (business/commercial) contexts. It encompasses modifications to corporate intranets (referred to as social intranets) and other classic software platforms used by large companies to organize their communication, collaboration and other aspects of their intranets. Enterprise social networking is also generally thought to include the use of a standard external social networking service to generate visibility for an enterprise.
-- Wikipedia