Update: The problem is easy to solve, the solution has been published in SNAP website, but a little hard to find between all the info. I will give you a quick solution:
In Facebook’s Valid OAuth Redirect URIs you must enter the URL of your SNAP/Accounts page. For me this is:
https://www.mysite.com/wp-admin/admin.php?page=nxssnap
Of course you need to enter your site URL in FB->App Domains (if you haven’t done this already).
Cheers,
Angel
