[Deprecated: Newer methods will do this without much work.] I tried to use the FB connect registration tool to create a single registration form. FB connect login works (well, the async code is pretty sketchy), but the registration code requires decoding a signed_request hash and, well, the code I was using to get the login info from the JS SDK wasn't getting the signed_request registration info. Oooff. That's a lot to say.
Anyhoo. Tried various examples, tried to use various FB SDKs with no love. User error no doubt. However, this example on Pastebin, Facebook Registration C# handling worked first time with no changes. Yeah! Thanks anonymous guest poster.