Be prepared for some pain when you try to keep Facebook login working in a captive portal environment. The web resources accessed by the login pages do change from time to time and you're going to need to keep your walled garden ranges up to date. You will find some resources online to help, but there's no substitute for testing and monitoring.
It sounds like you've selected a client-based login flow using Javascript (I'm partially guessing here). If so, reconsider that decision if possible. You're really setting yourself up for a world of future pain. I have found the server-to-server flows to be more stable. You also get a better place to monitor for user issues.
You should also be careful about your Facebook application permissions, as they control what data is shared with you.
On a related subject, make sure you do some testing with other Facebook accounts. You may find that permissions seem pretty loose when using your own account, if you also created the Facebook application and related tokens yourself. When using other accounts, you often need to go through Facebook's review process to be able to access the data you need.
Facebook WiFi provides for a pretty narrow use case. I'm not surprised you've decided to create your own solution.
Not sure if that's the information you were looking for. Feel free to ask if you have further questions. I've been spending more time with unique captive portal promotions based on customer behavior rather than on specific social login scenarios. My info may be a little rusty.
