SecureSocial redirects to the original page after authentication, but will do so with an 303 See Other response that'll result in a GET request on the target resource. Whilst it's a simplification to say you can't redirect to a POST request, it won't work with SecureSocial AFAIK.
A better way to handle this would be to prompt for authentication before the user submits the comment form, i.e:
- anon user wants to create a comment and clicks "add comment" (or whatever)
- do authentication via FB
- redirect back to comment form (GET), now with authenticated user
- proceed to process form submission (POST)
You can read about some issues with POST redirects here.