“[...] However, as a comment on the icons themselves, the green circle seems like it might be more appropriate for when someone uses the "Share" link to re-post something from someone else? [...]”
Well, the icon you see there depends on your system iconset.
It's the 'view-refresh' icon from the Freedesktop.org spec. In your iconset, it might be something else entirely, although they are usually some kind of arrow pointing to itself.
As Pump.io is now, at least, the 'share' activities don't appear in the minor feeds, so this particular confusion couldn't happen =)