Sorry for the delayed response. Facebook has recently updated their requirements to now require your site to have https (SSL) connections for authentication to work.
Adding SSL to your site used to be very difficult, but with tools like Cloudflare, you can enable SSL on your site for free, quickly and with additional features, like caching. If you don't want to use Cloudflare, there are plenty of other options, like purchasing and installing an SSL certificate on your own or working with your hosting company to install one.
SSL has many other benefits and most browsers are calling out sites that *don't* use SSL, so it's a good idea to have it setup.
I hope that helps, but if you need anything else, please let us know.
Thanks,
Alex