In the About the Technology section on the book's webpage it says "Hadoop powers mission-critical software at Apple, eBay, LinkedIn, Yahoo, and Facebook. What are some specific examples of how Hadoop is used at those companies?
Most of these organizations leverage Hadoop for some heavy data-lifting activities such as aggregation, structured and unstructured data joins, and data analytics. Hadoop is also used as a storage platform for places like Facebook where they leverage technologies such as HBase. There's a decent amount of literature online where these organizations have published presentations on their Hadoop use cases, which are probably worth investigating.