Late in January this year, Facebook introduced the ability for users to browse Facebook over HTTPS. As a result, the social networking company provided "Secure Canvas URL" and "Secure Tab URL" fields in the Developer App for developers to serve their apps through an HTTPS connection. Without these fields, users will see the following dialog when they visit your app:
Today, 9.6 million people are browsing Facebook over HTTPS and the trend is continuing to increase.
If you haven't already done so, please acquire an SSL certificate and fill out the Secure URL fields. This will ensure that users browsing Facebook over HTTPS will have a great experience over a secure connection.
[Source: Facebook Developer Blog]