It looks like you're new here. If you want to get involved, click one of these buttons!
I just downloaded HTTPS Everywhere for Chrome in a recent rash of suddenly caring about Internet security, and I found this out:
HTTPS Everywhere can protect you only when you're using sites that support HTTPS and for which HTTPS Everywhere includes rules. If sites you use don't support HTTPS, ask the site operators to add it; only the site operator is able to enable HTTPS. There is more information and instruction on how server operators can do that in the EFF article How to Deploy HTTPS Correctly.
And from just browsing online, it doesn't look like a great deal of sites actually support HTTPS on the server-side...
Comments
Tumblr doesn't support HTTPS. Sadface. Then again, it's Tumblr. They don't implement things that make sense.
You could use HTTPS Finder. That's kinda more of an addon to HTTPS Everywhere, but it's useful for sites that have HTTPS but aren't supported by Everywhere.