FWIW, ~33% of all pages are loaded over HTTPS already in Firefox, according to the metrics collected by the browser. And, over 50% of all HTTP requests (including subresources like images and scripts embedded on a page) are over HTTPS, again according to the usage data collected by the browser.
I'd be willing to bet quite high all these HTTPS requests are all on the same specific services (facebook, gmail, google would already take most of them).