This is only a suggestion. I suggest to always use https to embed sharing buttons, especially Facebook and Twitter buttons.
This is mainly because if you are browsing from a specific country/ISP, Facebook and Twitter may be blocked by Internet Providers if you are requesting them using http, but not https. Regardless, isn't it really better to always use secure connections? ;)
You may be using relative protocol, but my suggestions is that it's still better to use https always.
Thanks for the great plugin :)