Living in Syria, I feel really unhappy when a (Facebook, G+, Twitter... etc) plugin doesn't work on 90% of the web.
The problem is that these (social) websites are not welcome in Syria (gov decisions), but still work perfectly using https. However, because their plugins use relative protocols, and most websites use http, then these plugins will eventually try (and fail) to load using http.
The question is: Why ever use relative protocols if you can use https?, isn't it always better to use https and have your users' data transferred securely?
I don't think giant websites care about https overhead, so what am I missing about the whole thing?