Microsoft has announced its new Facebook SDK, which allows developers using Microsoft technologies to quickly create web client (via Silverlight), web server (via ASP.NET) and desktop (via WinForms and WPF) applications that harness the power of the Facebook API.
There isn't just one social graph, there are many. LinkedIn maintains your professional contacts, while Facebook has friends and family (and yes, often many professional). Twitter has some mix, as well, perhaps with a different filter. Aggregators take these disparate contacts and bring them into one place. A new service uses several APIs to merge them and give tools to better understand and utilize your network of contacts.