Fluid Topics has to be declared in the Facebook Apps Dashboard.
Once the web application is created, the callback URL must be set in the Advanced tab > Security section > Valid OAuth redirect URIs.
In the Status & Review tab, the application must be made available to the general public.
This realm's type is facebook. It requires two mandatory parameters: key and secret. Both of them are obtained when declaring the application.
Here is an example of a Facebook realm declaration: