Mike Gerra authored an in-depth report on Facebook and its impact on the online travel space. It can be purchased via Phocuswright.
The synopsis from Phocuswright:
The online social network space is one of the most dynamic areas of business and technical innovation on the Web today. Almost weekly, we come across a new social network and hundreds of new applications and services are launched on existing social networks every day. Over the last three years, two networks in particular have gained enormous mind share and market share: MySpace and Facebook. As consumers have swarmed to these networks, advertisers and businesses have raced to monetize the growing virtual neighborhoods. More recently, software developers have begun to leverage these communities, with their inherent viral and network effects, as launching pads for a vast array of innovative and attractive applications.