Well the namespaces have been recapitalized at the least. facebook -> Facebook. But the DLLs are also different.
FDT 2.0 binaries: facebook.dll, facebook.desktop.dll, facebook.web.dll, Microsoft.Xml.Schema.Linq
FDT 2.1 binaries: Facebook.dll, Facebook.Controls.dll, Facebook.WebControls.dll and oddly a sample app (FacebookDesktopSample.exe)
In 2.0 code: I can do this new FacebookService().API by only including "using facebook.Components"
In 2.1 code: "using facebook.Components" is now "using Facebook.Compontents", but the API property can't be found.
2.1 is my first use of FDT, but they seem like two totally different APIs.