G+ and FB show other images than what we intend to be shown when sharing. The share buttons are located at any confirmation page. According to FB's "Sharing debugger", it is just that the defined image is too small. This bug must have been sitting in our code for quite some time...
Currently it does not seem to be possible to determine whether people were referred by a post on G+ or on FB, since both use open graph. The referral keyword is set to FB, though.