IMPORTANT INFORMATION REGARDING FB APPS
Starting October 1, 2011 Facebook will require a valid SSL Certificate for all pages and applications hosted outside Facebook. But What is an SSL Certificate? Basically, it’s a security certificate. SSL stands for Secure Socket Layers, but it’s really just a way to encrypt digital information. Having an SSL certificate means that your website protects a user’s private information. Banks and online shops, for example, have SSL protection so your account and credit card numbers are protected.

This means you will need to upgrade your hosting plan to include this https feature in the domain where you have hosted the FB templates. After that, check the following section to see how you must config your app (for already users, you will just need to add the secure URL fields with the proper url path, starting always by https://)

 
HOW CAN I SETUP MY APP SETTINGS?
Check the following image to see how you must setup your APP, including the (now mandatory) secure URL fields. Remember you will need to add the SSL certificate on your domain. Contact your hosting provider for more information. Average prices for a SSL certificate should be around $50-$100/year, depending on your hosting pricing plans.

(note: new mandatory fields marked in red)

More info about FB APP Settings here: https://developers.facebook.com/blog/post/542/
More info about SSL certificates: http://www.sslshopper.com/what-is-ssl.html
Also, you may consider to check this great service before going any further on SSL certificates on your server:http://www.social-server.com/index/