FaceBook SSL Requirements
From the 1st October 2011 FaceBook requires all Apps, elements and page tabs to use SSL so they can be accessed via https.
If your App or Page Tab does not use SSL then you are at risk of having it disabled. You can purchase a RapidSSL for as little as $10 a year from ServerTastic. This certificate is perfectly adequate for use within FaceBook. If you have users who access your app via mobile devices then it may be worth considering purchasing a QuickSSL Premium certificate. This certificate supports older mobile phones. However I would argue that the majority of users accessing FaceBook via a mobile will be using a modern handset and therefore this should not be a problem.
The certificate is installed on your website, not FaceBook. It should be on the domain where your App or Page Tab is hosted. You must then provide your Secure URL in the Development App settings to avoid it being disabled as per the screenshot below.




