The steel-blue thin circle is unshared. It's meant to be indicative of the user's apparent disinterest in the matter. It doesn't stand out and doesn't take a lot of space. (too thin?)
The cyan thick circle is shared. It's meant to be bold and bright, indicative of interest. Its filled internal space is meant to be evocative of the radio button, like in one of those single-choice tests or surveys.
The orange full circle is self-post. It's meant to represent a non-choice that the user has over sharing the post: they can't, because it was already posted by them, so they already have.
The whole thing is meant to be simple, non-distracting without being bland and unappealing.
I had to fix the line height on the title lines to fit with the circle height. Currently, the circle is about 150% taller than the title line and is hanging over the author-and-date line.