Klout Score is an indicator of social media influence (for companies or individuals). Taking into account Facebook, Twitter, LinkedIn, YouTube, and other forums, it provides a score between 0 and 100 which is based on a number of (constantly evolving) criteria including the size of your network, the level of interactions.
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.