Just the other day I joined Airbnb in the hopes of trying to find temporary accommodation in the bay area and the first thing that hit me was the fact that I knew no-one off the top of my head with past experience to whom I could direct questions and the like. Then I noticed that there was a simple "X, Y and 23 of your Facebook contacts like Airbnb" at the top of the page which immediately helped the situation. Seeing this sort of integration go even further is a very cool use of social networks. Kudos to the Airbnb team.