The SSL requirement is from Facebook itself when using the "Requests 2.0" API. That's what we use and it therefore does require an SSL certificate on your site. We're investigating some ways to get around that requirement, but for now, you'll need an SSL certificate to use the Invite/Request feature.
SSL is also required for Page Tab and Canvas integration, if you want to show your site within a Facebook Page or Canvas area. Facebook requires the SSL certificate so that private data, like the viewer's Facebook ID, can be passed to your site in a secure manner and not in the open.
As for obtaining and installing an SSL certificate, you can view our
SSL certificates for Facebook
guide which explains a lot. However, it's a generic guide. If your server company has their own information about getting an SSL certificate we'd recommend that. The only thing to note is that the https version of your site has to have the same domain as the non-secure version of your site.
Alex
invite frineds from FB only with SSL?
